That depends on your tank and what you want. Just as my opinion I would add 50 snails to start and maybe 30 scarlet reef hermits. Some people want nothing to do with hermits, but the scarlets seem to be much less aggressive than blue leg hermits which will spend their day killing snails and each other. Hermits kill each other and snails in the wild as well as in tanks, so believe it is natural and will occur. If you put shells on the bottom for them they will still do it, if you make sure they eat well they will outgrow their shells and kill for new ones, but if you don't feed them well they will kill for food. They are omnivorous and do not just eat plants. I have had much better luck with the scarlets, just stay away from blue legs.