It is estimated that there are about 300 billion galaxies in the universe.

This is determined by counting all the galaxies in a small (very small) portion of space and then doing simple arithmetic.

Here is an example. Say you wanted to know how many rain drops fall in one second on a football field area. The way you could do this is to actually count the number of rain drops that fall on one square yard (a tiny fraction of the entire field) and then multiply by the ratio of the area of the entire field to 1 square yard.

In this calculation WE USE THE COSMOLOGICAL PRINCIPLE which states that ON AVERAGE any piece of the universe is JUST LIKE ANY OTHER PIECE OF THE UNIVERSE. That is, at a large enough scale, the universe is homogeneous.

We don't know how many galaxies there are in the universe. We can only guess how many there are total by counting the ones we can see and figuring out how many there should be, but we can't see because they are too dim or far away.

If there are a certain number of galaxies that we can see, how do you think you would guess how many there are total?