Snails are dependent on your bio load, lights, tank age, etc
There is no x amount for y gallons
Honestly I would get a mixture of snails and avoid hermits
Go for astreas and cerinths and brittle star the exact numbers of which I would wait and see
I have in my 90g 15 astreas, 20 cerinths, a few peppermint shrimp, a serpent star, one turbo snail (bulldozes everything) and 2 cleaner shrimps and a tuxedo urchin my tank is almost always spotless