At any one time, there are between about ten and fifteen different lenses available in Snapchat. New Lenses are constantly being added and old ones removed and then brought back. Theme lenses are often added for special events and companies can pay Snapchat to have a lens included to promote their product.
Despite being one of the best things about Snapchat, Lenses are hidden behind a touch gesture. While almost everything else in Snapchat’s interface is handled by buttons, there isn’t one for Lenses.