We see the galaxies as they were however many years it took for the light to reach us.

Every time we look, we find something new because there’s both a LOT of stuff in space and because light from further away keeps eventually reaching us.

…but mostly it’s a very complicated guess with a lot of math based on what we’ve already seen. Truth is, we don’t know precisely how many galaxies there are, but there’s always a whole shit ton of them everywhere we look, so it’s reasonable to assume the number is pretty big. And getting bigger because, again, we keep seeing new stuff.