I have a couple nerites in my 20g and they are great algae eaters, but they will leave eggs everywhere. It is especially annoying on the glass. Personally, because of that I would go with a different snail like mystery snails. On how many, my 2 nerites couldn't keep up with the algae in my tank (slightly planted, lots of driftwood, and it does get direct sunlight as well as a nicrew light, so I might have an outlier tank). Currently I have 4 mystery snails in addition to the 2 nerites and they seem to keep all the algae at bay. You could try adding 4-6 snails at a time, see how they are doing and reevaluate every couple weeks to see if you want more.