You could prob do around 5-10. They work steadily but don't expect your tank to be empty overnight. The important thing is reducing the feeding and the DIY traps. I'd rinse and repeat the traps for several days until there are noticeably fewer, and then the assassin snails will help control the rest. When there are a bunch on the glass like that just get your net and scrap 'em off and dispose.
There are fish that you could get like loaches and such but it's best to just hit the root of the problem instead. Definitely do not get any clown loaches unless you plan to upgrade your tank later.
Good luck and let us know how it all turns out!