[tomb1]

A few comments:

Stick with 4 denominations. If you want to buy a fifth denom just to have have some variety, that's OK, but you don't need them.

Don't use too many of the lowest denom. Lots of people realize this after a while -- if you have too many smalls (your $25s) in your starting stack then you waste your money on buying a lot of them, and end up with lots of useless chips once you color up the smalls. Use your smalls for blinds (starting at 25/50) and maybe limper bets, but your medium denoms get most of the action.

Let folks start with a nice playable stack -- maybe 25-30 chips. With only 20 players, the final few folks will have big stacks at the end, even after you race off the $25s and $100s.

You can easily have a 20-person tourney with 650 chips. We run a regular tourney with a nice T$5,000 starting stack:
8 x $25
13 x $100
3 x $500
2 x $1000
Total 27 chips, starting stack T$5,000

You can buy 650 chips like these and never run out.
160 x $25
300 x $100
110 x $500
80 x $1000
Total 650 chips

OR . . . you can have a really rich and really fun starting stack of T$10,000 and still only buy 800 chips:
8 x $25
13 x $100
7 x $500
5 x $1000
Total 33 chips, starting stack T$10,000

Minimum 800 chips (but I'd probably add more $1000s and buy 850):
160 x $25
300 x $100
200 x $500
140 x $1000
Total 800 chips