This tank will be on the small side, they would do better in one twice the size.
If you want a mixed group of males and females then I would say 3 maybe 4, you should have only 1 male with 2 or 3 females. This will leave you no room for fry, how large do you intend to grow the fry before re-homing them?
You might do better with only males, 4 or 5 as they are smaller than females, or if you want them to have fry, how about a couple of females who are already pregnant? Most will be pregnant when you buy them from your LFS, they could then have up to 7 batches of fry without a male in the tank, if you do this then you should have enough space to keep 1 or 2 of your fry.