The exact number of stars in the universe is not known, and the total figure might be unknowable from the Earth's position in space. The universe almost certainly contains no fewer than 10^24, or 1 septillion, stars, and it might contain an infinite number.
The Milky Way is an average spiral galaxy with a mass of approximately 100 billion Suns. Since most stars are less massive than the Sun, a reasonable approximation is that the galaxy contains around 400 billion stars. The observable universe contains approximately 170 billion galaxies. If the average galaxy is like the Milky Way, the observable universe contains at least 1 septillion stars. The universe might extend endlessly outward, however, in which case the available volume might contain infinite stars.