2 fish would be fine in a 100 gallon. however, make sure you choose a hardy fish. Do not start out with any tangs or angels. Personally i hate damsels, but they are a hardy, cheap fish.
So you cycled with a damsel? So he made it through, so now you are stuck with it, or you can trade it in towards another fish, I wouldn't keep it, but thats your choice. You can add 2 fish with that size tank. Maybe afew chromis. I put in 3 plus a cleanup crew in my 150/30 gal fuge.
I would only add 1 every 3 weeks, unless it is a schooling fish (chromis) or something that you need to add in pairs or a trio (clowns or anthias).
By adding one every 3 weeks, you allow plenty of time for the biological filter to build-up up and establish enough bacteria for each new fish.