40 breeder. How much rock work. How mature of coral colonies and what type(s)? Makes a big difference as to how many you could add. Typically we recommend damsels in the chrysiptera genus. Example would be Azure, Talbot, Surege, Rollands, etc. Plenty in that genus that have good colors and reports of being acceptable in a reef.
The challenge comes down to how many and if they form a pair. Spawning can bring aggression as would protection of the nest. I've kept 7 Sapphires in my 210 gallon mixed reef but they are too protective of the nest, eggs. Once pair was fine but when two pairs started to spawn it was too much chaos.
Basically if you have plenty of rock work and larger corals it will work. They need place to sleep and things to break up a chase. If more than one and a pair forms they need nesting areas. The more of this that you can provide the calmer things will be. If not then protecting territory happens and you will suffer a loss.
It may be wise to add the damsels as your last fish. This way should they form a pair you don't have to worry about new fish getting picked on because they don't know the lay of land. Which rock or coral is what fishes sleeping or nesting area...
I currently have 11 Biota Lyretail damsels in my 210 display and it is a lot of fun.