IMO, there is no exact amount...... Some hermits and snails, types dont matter so much withhermits, but different snails may act differently. like a mexican turbo will eat a lot more than a margarita. and a tongan nassarius will bury in the sand to clean the bed, but that is not recomended for such a small tank. To tell youthe truth, i havent gone out and bought a clean up crew for either of my tanks (29 gallons and 40 gallons), i started out with about 5-10 hermits and each time i go to my LFS i get another one or two. same thing for snails i have a couple in the 40, but i think my puffer keeps eating them in my 29.
But if you are looking for a guide or a package to buy, the nano or even the 20-55 gallon pack should work out nicely. However, brittle stars, im not sure if they are green oens or not, can become predetors once they get big. This goes the same with the coral banded shrimp. However some are perfectly docile and friendly. It all depends if you want to risk what is stated above if you are looking to buy from this site IMO.
PS: stay away from sand sifters because they will eat up all the creatures in your sand bed of a tank taht size. Also coral banded shrimp need a lot of places to hide, so if you dont have too many, and somefish taht like to hide also, that may not prove to be a good thing once the shrimp gets large.