You asked how many fish you can have in a 90 gallon tank. The answer is, it depends.
Your husbandry methods matter. How much do you feed? How much filtration? Do you do water changes early and often? Obviously, a very zealous husbandry will allow for far more fish than lax methods.
Also remember that there is a margin of error. If you put every last fish into a tank that it can possibly hold even a small degradation of conditions could cause a catastrophic crash of the system.
Fish really matter more on their demands on the system. Very active fish have higher metabolic rates. Large fish make more demands than small fish.
There is even the interaction of fish to be considered. I have sen fish that were very aggressive in a tank that became quite peaceful in a larger tank.
So when you think it through, you are talking about not just a simple thing like system gallons vs number of fish. You are talking about a multiple variable situation.
Probably the best thing is to plan your tank fish list on the conservative side. Add fish slowly and see how the tank parameters hold up.