Most tournaments geared for bad-beat type re-buys/re-entries have a re-buy rate between 25% and 33%. Budgeting for 50% re-buys is sufficient chips to cover nearly all scenarios, especially if you have already budgeted extra top denominations in the set for occasional larger-stack events. Re-buy rates only exceed those above for very fast or very inexpensive turbo-style tournaments where loose play and multiple re-buys are strongly encouraged (like charity events, or $5/$10 T5000 events with fast blinds).
You won't need any extra blind or first-workhorse chips for the re-buys/re-entries; just use the second workhorse chip and color-up chips instead (and let players get change from the chip leaders).
I was thinking of a T20K tournament with an 8/8/6/6/2 starting stack. Open to other suggestions though.
I'd suggest 8/8/4/7/2 stacks as a minimum instead,, since more T1000s (and fewer T500s) create a better breakdown of chips on the table for actual play. But actually, I'd really recommend 12/12/5/6/2 starting stacks, but that's a different debate -- but it would be much better, especially if you are planning to structure re-buys/re-entries with larger chips (so that you have enough lower denomination chips as change on the table to break them down).
At a minimum, allocate 5xT1000 and 3xT5000 for re-entries (times 50% of the field size), and use T1000s to color-up the T25/T100 chips. Have enough T5000s on standby to color-up the T500s. If you can afford 20 extra chips, then go with 10xT1000 and 2xT5000 for the re-buy/re-entry stacks to make things easier on the players from a change perspective.
For a 10-player single-table T20K set (12/12/5/6/2) with re-buys/re-entries, I'd recommmend:
120 x T25
120 x T100
50 x T500
125 x T1000 (includes 15x for T25/T100 color-ups and 50x for up to 5 re-buys/re-entries)
40 x T5000 (includes 10x for T500 color-ups, 10x for up to 5 re-buys/re-entries, and 5 extras)
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455 chips, using 10/2 for re-buys (get extra T5000s if you want to round up)
~OR~
120 x T25
120 x T100
50 x T500
100 x T1000 (includes 15x for T25/T100 color-ups and 25x for up to 5 re-buys/re-entries)
45 x T5000 (includes 10x for T500 color-ups and 15x for up to 5 re-buys/re-entries)
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435 chips, using 5/3 for re-buys (get extra T5000s if you want to round up)