Well, I personally would say 6 (3 pairs) since the footprint of the 75 is the same the 55. They like to stick mostly on the bottom, so the upward space isn't as useful to them. Maybe 8, but I think 6 would be a good healthy number because then if they all paired up, each pair would have a good sized territory. Of course, it also depends on tankmates and stuff. They seem to do best in pairs if you are doing multiples, but it is not really a big deal as long as there is space for everybody to set their own territory. You could start with 8-10 juvies and see who pairs with whom and how they get along and perhaps sell any outsiders to an lfs.
Personally, I like to make sure my rams have a lot of space since they are sensitive, so I do not want them to have any stress due to potential neighbor issues.
