What type of hermits? I wouldn't add any more than what you have.
I view hermits as "optional" for the reef. They do scavenge for food and contribute to the clean up crew. However, they often will kill snails (either to take their shells or not) which gives them a downside. Conventional wisdom is that blue-legged are the most likely to kill snails. I had 4 in my 37g reef and 3 of them ended up killing cerith snails and taking their shells (despite many other similar sized empty shells in the tank). I have a single scarlet hermit in my 12g now.
As far as snails go, nassarius are good, but they don't eat algae. For my 12g nano, I have 4 nassarius. 2-4 is a good number. I would recommend some algae eaters. If I can find them locally, I'd like to get 4 cerith and 2 astreas for my 12g. I have had good luck with those in the past.
A single turbo would be a good choice too. Margaritas are not a good choice because they are really sub-tropical (need colder water). Bumblebee are supposed to be bad too but I don't remember why.
