That would depend on what genus and species you're calling a damsel.
Though to answer your question, no more than two but more than likely one in that size tank.
I have 4 Damsels (Domino, yellow tail, white tail, and the other is a white and brown ugly mofo)...
I have 2 of them in the 10 gallon (They seem ok) but I have 2 others have to take out of my 30 gallon since they are starting to get aggressive for my clows and the others in there ...
I would bring them back to the LFS but the girlfriend likes them to much... :roll:
I guess I'll have to tell her the bad news
And another question... Are damsels to aggressive to to mix with a cleaner shrimp?