In the observable universe there are about 80 to 100 billion galaxies with varying numbers of stars in each. A rough mean average for the number of stars per galaxy is 400 billion. It is thought that there are from 30 to 70 billion trillion (i.e. 30 to 70 sextillion) stars in the observable universe. Whilst the estimates differ somewhat, the number of stars seems to be in the sextillions, which is a 1 followed by 21 zeros.
There are many reasons why our estimates vary as much as they do. An important point to consider is that when we observe distant objects such as stars, we don't see them as they are now, but as they were in the past. The reason we see into the past as we look out into the universe is because the light we see takes a finite amount of time to reach us. As such, some of the stars we see, particularly those in very distant regions of space (relative to earth), may no longer be counted as stars. Conversely, new stars in distant regions may have already formed but we wouldn't see them until light from those stars reaches us here on earth.