Years of experience/online research/talking with fellow hobbiest. Less can do a good job, but 1 per gallon is a good benchmark, IF you are attempting complete algae control using only nerites.
As I mentioned, I recommend a good mix. A 45g tank with 45 nerites would really look pretty crazy, unless you just really love snails.
I see. It's just that I have never seen someone recommend so many, even people like msjinkzd states one per five gallons.
I suppose it would depend on the amount of algae present, however.