How Do We Know How Many Stars Are in a Galaxy?

How do we know how massive, and that they're bigger than they should be? Like, just from the size of the mudge or what we can tell by how bright they are. The more stars that there are there, the more photons we're going to get. That's a way to tell how many stars there are. And this depends on whether or not they're still making stars, and how old the stars are in them.