I would say in a 55G the Flasher/Fairy wrasse family is the best option. Probably the most jumpy of the wrasses though. You can do a couple leopard wrasses as well but those are considered "Expert" fish, although the term expert can mean a lot of things. I think in the terms they are using it means your tank is well established and you have a firm grasp on the requirements for the fish.. other than that there isn't a whole lot you can do other than try and cross your fingers. Stay away from Coris wrasses, even most Halichoeris can be a little on the agressive side compared to flasher and leopard wrasses.
As for a guess on a total.. I would say 4-6 wrasses in a 55G if they are suitable tankmates.
I saw them for the first time about a year ago at my LFS but they were not that common, but common enough that I could ask for one and I could get it within a month. Problem was my LFS was not having much luck keeping them alive/eating so I never got one. I saw one today at a different LFS and it was very active and hunting around the tank and it ate food in front of me so I grabbed it right away. Floated it/acclimated it in my tank and let it go and it has been acting great since it went it. Hunting and picking at the rocks and swimming around really good without anyone bothering it so I an hoping I got a good one. 
