for a 75g assuming your planning to have otherfish... I would do atleast 20 loaches if you love them that much as I do. even with 20 loaches ranging from 3 to 4" that would be a joke to a 75g. Its really a very broad question though.... cause if your able to provide more filtration than needed and your willing to do more frequent wc than you can put a LOT more loaches in a 75g but if you dont have taht much time to do maintenance than you shouldn't keep too many. The thing with Clown loaches is that the more you have the more crazier they get =D and its soo much fun watching a HUGE army of loaches having a party everyday!
So if I had a 75g with no other fish.... I would probably do around 50 loaches ranging from 2" to 4"+ with an a lot of filteration and daily WC with Aged water (atleast 24hours w/ Prime).