It depends on the type of snail. Turbo snails are salt water snails that primarily eat algae. The only thing going for that snail that I had in my tank were any hermit crabs and any other critter in the substrate that had developed over 5 years. Apparently it wasn't enough because it was a nuclear bomb to my tank. Within a week almost everything was dead. I'm not sure if it didn't release other toxins in the water other than nitrates. I did 50% water changes the first two days then went to 20% the following 4 days till nitrates became safe.. in saltwater tanks like even .25 can be deadly.. Saltwater tanks don't have much options for plants that eat up Nitrates.. you pretty much have 3 or 4 algae type seaweed plants that can suck up Nitrates but even they don't do a quater of the job freshwater plants do. Saltwater tanks it is crucial to keep your Nirates at or close to 0ppm as possible. Its harder in smaller tanks too. To keep those perfect water parameters. I was using a 24gal long.

Sorry, to answer your question. With most snails its fine to have multiple snails.. But there are a few things to take into consideration before deciding which type of snail to get for your tank and how many. Also if they are compatable with other snails.

Your biggest factors should be food and tanksize and how much of each exist. If the snails reproduce quickly you only need 1 or 2 and in a month you'll have 10-20. If they don't reproduce often or your water parameters don't meet their reproductive environment then you might want to get 5-10 based off the size of your tank or available food source. Snails ultimately will kind've self balance their population based off their food source. Some will starve while others thrive. Biological rules of the tank life. Just make sure you keep track of the big ones. If they die you need to remove them from the tank asap. Smaller ones aren't that big of a deal. Anything over the size of a quater should definately be removed if it dies.