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

Well there are over 125 billion Galaxies out there that are estimated of at this time. Each holds anywhere from 100 to 400 billion stars each. That is a lot of starts! Hope this helps. :)

A galaxy is a gravitationally bound system of stars, sellar remnants, intersellar gas, dust and dark matter. 
There is an estimated 100 billion galaxies in the universe or so but this number is likely to increase to about 200 billion as telescope technology in space improves. 
Each galaxy has between 100-400 billion stars