There are probably something like 200 to 500 billion stars in the Milky Way galaxy with the majority having masses from 0.5 to 0.01 the mass of the Sun; and very difficult to detect at great distances in our own galaxy. If there are 50 billion galaxies in the visible universe out to 15 billion light years, then there may be something like 2500 billion billion stars. I personally don't have much faith in such an answer which is uncertain to probably a factor of 100 or more. Any number between 1000 million billion, and 20000 billion billion could be plausible.