How many stars are there in the entire universe?
No one knows, they don't ever dare guess.
Consider that the average constellation, like the Milky Way, contains as many as 3 billion stars. Now you have to multiply that by the approximate number of galaxies which also number in the billions.
And time gets in the way. We can only see about 13.4 billion light years distant. Most astronomers think it entirely likely that the universe extends to at least 100 billion light years and may actually be infinite. And so they do not ever try to guess.