I'd go with 12/12/9/X. So for T10k for 2 10 player tables you would need
because there's 200,000 in play and the tournament could reach a level where you only have the T2000 left, so you need 100 of them (actually more since color-ups tend to add to the chip count).
For T15k stacks and above I would add T10,000 chips (which would mean that you no longer need more than 100 T2k chips).
With rebuys you need even more T10k.
T50K chips won't be needed unless you'll have ~ a million in play.
This was not entirely correct. For T15k it would work with 12/12/7/5 since with 20 players that's 100 T2k, but you'd need more to cover color-ups. For T20k stacks you'd need 12/12/9/7, so you'd need 140 T2k. In both cases I would get some T10k to handle color-ups.