they don't actually have an optimal number, honestly anything over 5 is good, but the bigger the tank, the more you can have (clearly). I used to have 6 in my 75 but they weren't very big. 6 CLs can outgrow a 75 quicker than you'd think.
How many depends on their size. My 4 biggest 8" to 10" wouldn't fit in a 75G, but I have >40 4" to 7" in my 300G. About 15 to 20 <4" CL would be a nice group in a 75G. Remember they grow slow until about 6" and may stop growing (perhaps even sooner in a 75G).
Knowing that you are going to upgrade soon, I would go for at least 12+, but that is just my opinion. I have 36 in a 220 and they have plenty of room to play and goof off. The more you have the more activity you will see. And believe me you will see some crazy activities coming from them. Lots of fun!!!