I have about 110 in my 90, and 17 in my 10. I think the rule is minimum of 1 lb/gallon, and a more is always better until your tank looks too cluttered. Look for really porous rock, as there is more surface area per pound for your beneficial bacteria to adhere to. I had 50 in my 50, and I could certainly have done with more. When I set it back up, I'm looking to put in about 75 lbs. Some will be in the sump, but the majority will probably be in the DT, as my stand doesn't allow for a large sump. So keep adding rock until it looks too crowded!