I have 14 in my 210 right now. Of those 14, 5-6 are still juvies (under 5 inches). I have to do large 75%+ water changes every 5-7 days to keep my nitrates down. To keep my water quality in check once they are all grown I would doing massive water changes aver 3 days or so. To qualitfy my statement I will add two things. One, I like low nitrates...less than 20-25 ppm. Second, there is no "rule". Heck if you are up for the water changes then you can put in as many as aggression and your ability to keep the nitrates low allows. My plan once they are grown in to have 8-10 in my tank.