You could go with 8 or 9 discus total for life in a 90 gal. safely. You could even go more if you did daily or every other day water changes, though I hesitate giving advice that people overstock their tanks. Not everyone is as anal about water changes as I am. I have 27 adults in a 240 gal., 15 in a 125 gal., 18 juvies in another 125gal., and so on. If you overstock and miss water changes, your fish will suffer.