I agree with Laura about having two and also that they may not eat the stringy algae, but definitely near every other algae. Also I would keep in mind that nerites are a bit different than the other snails, once they clean your tank and run out of algae it's possible that they can die off from lack of food whereas the other ramshorn, apple, or mts can pretty easily survive without a nice supply of algae. Nerites also do better in harder water, soft water can soften their shell in some instances.
Oh and when you do get them and start finding white spots every where, you may be like me and totally freak out, but it's just their eggs they are leaving around that you can scrape off, since they don't reproduce in freshwater, just lay eggs.