I think 8 would be a good number to have in a tank that size and as Ozgrrl said just make sure you have lots of places for them to hide.
Mermaidmel: i have 12 females in with rosey, lemon and silver tip tetra and they all seem to get on just fine in fact they are in with lots of other community fish and i have never had any problems but then i do have lots of places for them to hide too