Too many for the human mind to comprehend. If you take the universe as an infinite space, then the number of stars would be infinite. Its worth noting that the number of stars will be changing because stars are formed and eventually 'die'.
Countless... there are far to many for anyone person to comprehend, as Taylor said. If one were to attempt to count, they would soon be lost - there are far to many constellations, forever changing as stars would be born or die, and new ones would come in the place of old ones.