How many stars are there in the known universe?
Very rough estimate: 10 trillion
David Kornreich gave a rough estimate of stars.
But if you do this mathematically, by multiplying the average amount of stars (ranges from 100 - 400 billion) in a galaxy with the number of galaxies (100 - 200 billion) in the observable universe. The number ranges from 10 sextillion to 80 sextillion stars.
But the NEWEST estimate shows that there are 10 times more galaxies in the universe, so you have to bump those numbers up by an order of magnitude.
However, all of this is a very rough estimate, and the average galaxy is defined by the Milky Way, and we are not even sure how many stars are there in our OWN galaxy.
Hope it helps.