How do astronomers estimate how many galaxies are in the universe?
They typically count the galaxies in a very small region of the sky and extrapolate.
There are other moethods including estimating the mass in a cluster of galaxies from their gravitational effects and multiplying up.
The point is nobody can possibly count all the galaxies, all figures are estimates that improve on previous values. It is worth noting that only 100 years ago astronomers thought the Milky Way was the whole universe .... this is a very young science and so is a very exciting area to study.