In my 25g tank I slowly added snails over the course of about 9 months until I found the balance. The balance I wanted is for most, but not all, of the algae on the glass to be gone every day. That is the only way I know of to be able to determine if there is enough food for everyone. I never lost a snail, other than shortly after acclimation.
In the end I had 4 Nassarius vibex, 5 nerites, 4 Tectus spp., 7 cerith spp., and some other hitchhikers (Collinista and Stomatella, populations wax and wane).
That wasn't the end of the clean up crew of course, there were also hundreds of bristleworms, hundreds of brittlestars, uncountable pods, and many worms of all sizes. I also had a large peppermint shrimp for aiptasia, I got lucky, he is very effective.
Nassarius: The only whelks that are obligate scavengers. All others, including similar looking ones might be scavengers, but they are also predators.
Tectus: You won't find this name in any stores. They are always mislabeled. They are able to right themselves and eat lots of diatoms and microalgae.
Nerites: Most species are very good eaters of adherent microalgae. I have seen some inappropriate intertidal species in local stores, you can sometimes tell as some will be well above the water line, or even on the outside of the tank.
Cerith: Detrivores, bactervores, algavores, sand cleaners. Once you put them in your tank you won't see them again, except for once a blue moon.