How many neons can i add to my 20 gal?

I have a 20 gal as u may already know. I have 2 painted tetras and 5 neons. Can i add more neons to the tank or am i already at a limit with the fish i have. Today i went to petland and saw some painted tetras was considering buying one but then i saw one with only an eye. I decided not to buy it. I was also thinking of adding another kind of tetra and adding some bottom feeders to help with algae. Any suggestions will be greatly appreciated. Thnx.