What is a galaxy? how many galaxies are there and how many stars does each galaxy hold?

A galaxy is a large system of stars, remnants of dead stars, dust,
interstellar gas, and dark matter, all related to each other by gravity.

Within the part of the universe that we can observe, it's estimated that
there may be as many as two trillion galaxies !

Galaxies come in all sizes.  Tiny shrimp galaxies hold no more than
a few billion stars.  Really big galaxies may hold 100 trillion stars. ( ! ! )

The galaxy we live in (the "Milky Way") is somewhere in the middle.
It's very hard to estimate the number of stars in it, because we can't
get outside of it and look it over.  Estimates of the number of stars in
our galaxy range from 100 billion to 400 billion stars. ( ! ! ! )

Liquid. good luck : )