It depends on whether you want the big winners to accumulate money, or stacks. Some people have a huge affinity for building stacks; this may be especially common among chippers, but it doesn't appeal to me quite so much in game. I realize that part of it is the appeal of feeling like you're in a casino or poker room, where people keeping walking up with fresh chips, so a big winner accumulates a lot of stacks... and the other part of the appeal is just having stacks and stacks. I'm just not that into it as part of the game, but to have those chips in my collection... mmmm....
There also exist the opposite: people that crave coloring up. For them, the big chips are like "points." For them, making change for someone else is proof of being a winner. There's one guy I know who, in a $.50/$1 game, can have six green $25's and then twenty dollars in 1's and 5's... and he'll play like he's practically felted, because he wants to only playing with the small chips. If you push the bet to $20, he'll tank like you've put him all in. It's comical.
Personally, I just want to know what I have, and what others have... I'm a numbers guy. And I want enough chips for the game to play well. Whoever is losing will ALWAYS run short on chips and have to break their bigger chips, but change-making is trivial. In fact, all my regulars have learned to make change out of the pot, and we very rarely see people making change for each other. It's pretty much a non-issue.
With that in mind, this is how I'd build up the breakdown for you:
First, make sure there are at least 20 of your "action chips" for each player you want to be able to buy in; then, make sure the bank is big enough for all your re-buys by adding bigger chips.
24 players times 20 chips is 480. So start with:
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]0.25[/TD]
[TD="align: center"]120[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]2400[/TD]
Total Bank: 3000, Chips: 1440
The provides the minimum action chips in all denoms. Is the bank big enough? In my games, the average big-stack buy-in for 1/2 is under 200, before rebuys; if everyone bought in for 200, it would be 24*200, or 4800. I prefer 25's, but you're using 20's... so you'd need 90 of those:
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]0.25[/TD]
[TD="align: center"]120[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]2400[/TD]
[TD="align: center"]90[/TD]
[TD="align: center"]20[/TD]
[TD="align: center"]1800[/TD]
Total Bank: 4800, Chips: 1530
This is probably all you
really need right now, especially since you said your game won't garner 24 players when playing 1/2. But if you did fill 24 seats, you'd probably have a fair number of rebuys to deal with. Now, not everyone will have bought in for 200 - many will come in for 100 or whatever - and those are more likely to rebuy. Also, not everyone will rebuy; those who get ahead may never have to. So if you just double the bank, you probably cover all the rebuy action on a given night.
If you could imagine filling all 24 seats at 1/2 - or, in a few years, getting a dozen people playing 2/4 - you should double the total bank, which you can do by adding another 240 20's:
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]0.25[/TD]
[TD="align: center"]120[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]2400[/TD]
[TD="align: center"]330[/TD]
[TD="align: center"]20[/TD]
[TD="align: center"]6600[/TD]
Total Bank: $9600, Chips: 1770
This is almost certainly all you really need to have good playable games at all the stakes you listed, even if you have 24 people dealt in.
The remainder of the purchase is optional, and depends on your goals. Do you want this to be your legacy cash set, that you're playing with 50 years from now? (Or that you've bequeathed to someone, who is playing 50 years from now?) Then you should expect to be playing 5/10 or maybe 10/20. Fill the set with 20's, or else you've built what will one day only be a micro-stakes set. Eight big-stack buy-ins at 5/10 need an 8,000 bank before re-buys. At 10/20, that's 16,000. To future-proof, you'd want to fill in the set with 20's.
Option 2: Future-Proofed
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]0.25[/TD]
[TD="align: center"]120[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]2400[/TD]
[TD="align: center"]560[/TD]
[TD="align: center"]20[/TD]
[TD="align: center"]11200[/TD]
Total Bank: 14,200, Chips 2000
This is perfectly playable as a single table of 10/20, although a couple of 100 plaques will keep the bank from ever busting on a high-turnonver night.
The other option is to fill in with smaller chips, if you and your players are the type to enjoy collecting stacks. Honestly, you'll ALREADY have plenty of chips for stack collectors when you only have ten or twelve players, but if you really get 24 and you still want many people to be able to build pyramids, you may want to buff out the singles and fives. For example:
Option 3: More Stacks
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]480[/TD]
[TD="align: center"]0.25[/TD]
[TD="align: center"]120[/TD]
[TD="align: center"]590[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]590[/TD]
[TD="align: center"]600[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]3000[/TD]
[TD="align: center"]330[/TD]
[TD="align: center"]20[/TD]
[TD="align: center"]6600[/TD]
Total Bank: 10,310, Chips: 2000
There's also a strong argument that this setup still has too many quarters. You know your crowd, but if I had 24 people show up and play on three tables, there would almost certainly be at least two different stakes being played, if not three - .25/.50, .50/1, and 1/2. The quarters are only small blinds at .50/1, and are not used at all in 1/2. So I'd cut down the quarters in favor of more high denoms. Even if you really get 24 players of .25/.50 from time to time, I'd still cut the quarters down in favor of more headroom, perhaps like this:
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]400[/TD]
[TD="align: center"]0.25[/TD]
[TD="align: center"]100[/TD]
[TD="align: center"]600[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]600[/TD]
[TD="align: center"]660[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]3300[/TD]
[TD="align: center"]340[/TD]
[TD="align: center"]20[/TD]
[TD="align: center"]6800[/TD]
Total Bank: 10,800, Chips 2000
If it were me and my players, I'd cut the quarters back to 300 at most, which is where jbutler ended up. Note that his breakdown also got a lot of headroom from the 40 hundos, but you've said you can't get them as chips. I'm providing 340 20's to get 6800 of bank; his suggested break down gets 7200 of bank from only 200 chips:
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]160[/TD]
[TD="align: center"]20[/TD]
[TD="align: center"]3200[/TD]
[TD="align: center"]40[/TD]
[TD="align: center"]100[/TD]
[TD="align: center"]4000[/TD]
Total Bank: 7,200, Chips: 200
The hundos go a long way in giving you headroom.
In actuality, I have one 10-person hold'em table. My cash set has 160 $1's and 160 $5's - that's less than 20 1's per person in a 10-handed $.50/$1 game; on average, it's 16 per player, full table. (Though I buy in the first eight players with 20 each, and the late buy-ins come in all red, which is quicker and easier for me, anyway - at that point, I'm buying them in between hands.) At $.50/$1, we often have all the $1's and $5's in play, and it never felt like we're low in either denom on the table... even though there's usually someone with about 100 1's at the end of the night. I really think 20 per player is ample, and you'll get more long-term mileage out of the set by having more bigger denoms. If I were you, I'd work off of something more like this:
[TD="align: center"]Count[/TD]
[TD="align: center"]Denom[/TD]
[TD="align: center"]Value[/TD]
[TD="align: center"]300[/TD]
[TD="align: center"]0.25[/TD]
[TD="align: center"]75[/TD]
[TD="align: center"]500[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]500[/TD]
[TD="align: center"]500[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]2500[/TD]
[TD="align: center"]500[/TD]
[TD="align: center"]20[/TD]
[TD="align: center"]10000[/TD]
That's a Total Bank: 13,075 from 1800 chips. The last 200 chips I'd put into the 20's, for headroom, but if you want more stacks, you can put it into 1's or 5's, your preference. And I'd definitely get some matching hundos or hundo plaques - at least 20, ideally 40. With the hundos, this becomes a playable heirloom set.
I don't know if you have a separate tourney set, with 24 plaques, and 700 20's, this set has ample chips to do some nice tourney buy-ins for 24 players with just the 1/5/20/hundos. If you buy in with 20/20/9/1 of T1/T5/T20/T100, (total starting stack T400) and have 24 players, you put the following counts in play at buy-in: 480/480/216/24. You have 484 20's in reserve. If you hold back 150 20's for running off the 500 1's and 500 5's later, then you still have enough chips for one re-buy at regular chip counts, and sixteen re-buys as stacks of twenty 20's. If you don't want to limit the tournament to seventeen re-buys, you'll need four more plaques per re-buy.
And all that assumes you don't want to play the fracs in your tourney. (Some people hate that.) If you're OK with the fracs, you have all kinds of options.
Oops, just realized how long this post ran... I sometimes think out loud. And I type fast.