A dozen is too many. I'd go with no more than 1 per 10 gals (standard tank footprint, not volume), so for your setup, I'd go with 5 to begin with. I'd stick with scarlet reef hermits (Paguristes cadenati) as these are the most docile, remain small, and have the least tendency to eat snails or bug resting fish.
You need to stock your CUC lightly, esp. at first due to the lack of food in a new system. What ends up happening is your CUC will die back to meet the tank's needs, so why pay for critters that are going to die anyhow?
HTH