....What?
I would recommend 3 or 4 in a 90 gallon. If you are growing them out as juvies, then I would recommend getting 6 or 7. They will pick off the weakest of the bunch and you'll be left with the top specimens.
You sure sound like its guaranteed that the numbers will ween down. Its possible not for sure that "they will pick off". I started off with 11 juries and ended up with 11 adults and I'm sure there are plenty of other pkeepers that kept they numbers the same as when they started. Must show you must be doing some wrong to get your numbers down like that.