that's way more than I'd ever recommend. For a 90 gallon, almost all tangs are bad news (especially the hippo tang), and I'd only suggest one of the
Ctenocheatus spp. fish. And only one.

A foxface may be alright, but they get large and eat a lot. really you've got two things to consider, first is bioload and second is environment. I'd try to keep it under 10 fish total (better to be in the 7-8 range), and any fish that can get over 3" I'd count twice.
Pay heed to compatibility of the various fish you're looking at and ADULT size. I think 20 fish is wishful thinking, you'll need about 2 times the volume you're talking to pull it off with success over the long-term.