Depends on the size of gravel. They can do fine in any tank... I keep mine on bare bottom at the moment (till I switch substrate in a different tank) but they prefer to burrow, so sand is suggested. They are mostly used for arrating the sand so people don't get gas build ups, and aren't too useful for much else in my opinion since they are so small in size. If you do decide to go with them I think 25-50 would be a good start, they multiply like crazy.
But if I were you, I'd just go for some ramshorns, nerites, or mystery snails since they grow to a decent size and will do a much better job with cleaning (if that's what you wat them for)