It depends on the size/age of your fish, as well as the contents of the tank itself. In an average setup, I would guess that you could do 8 young adult (about 5” discus) comfortably. If they’re smaller you could probably keep 10 or so, but bear in mind that when raised and fed properly, these fish can max out at 9”+ each.
The decor is also a factor. If you are thinking about heavily stocking plants or wood, that will cut down on the available swimming space in the tank. I have a 75 gallon, but because my tanks are so heavily planted, it would be far too packed in there if I had more than my current 4.
When they reach adulthood, these gentle, docile fish can become rather territorial during their mate selection and breeding processes. That’s something else to keep in mind when keeping so many fish in a single tank, even a 100 gal. When you have 4 couples fighting for territory, a 100 gallon isn’t as big as you’d think.