I had the same exact problem in my ten gallon! I'd say a couple hundred at least...I put in 7 assassin snails, and *POOF* they were gone in a week or two..I know a lot of people say that assassins aren't effective, but I found them very much so. You just have to get more than one or two
I usually hate any kind of snail, but I love my assassins! They're really neat! And they breed VERY, VERY slowly...which is an upside!