I agree with FF - always better to start with less, and need to add more, than to start with more - and have them starve.
Personally, I'd forego the hermits altogether (or get very few), and get a good mix of different snails (different snails will work different surfaces -- rock - sand - glass -- and will eat different types of algae).
For a 75g, I'd initially start with about 20-25 snails (plus an additional 25-30 Nassarius & Cerith if you have a DSB
). Maybe 5-10 scarlet legged hermits if you like crabs. A Brittle or Serpent star is also an excellent addition (just don't get the Olive green brittle)