There are an estimated 100 billion stars in the Milky Way galaxy alone, and there are an estimated 2 trillion galaxies in the observable universe. This means that there are potentially around 200 sextillion (200,000,000,000,000,000,000) stars in the observable universe.
Keep in mind that these are estimates and the actual number of stars in the universe may be much higher, as it is difficult to count the exact number of stars, due to the vastness of the universe and the limitations of current technology.
Scientists use a number of methods to estimate the number of stars in the universe. One method is called "star counts," in which scientists observe a small patch of the sky and count the number of stars within that patch. They then use that data to estimate the total number of stars in the entire sky. Another method is called "luminosity function," in which scientists use data on the brightness of stars to estimate the total number of stars in the universe.
Another method is called "Extragalactic background light (EBL)" which measures the total amount of light emitted by all the galaxies in the universe, and then assuming that each galaxy has an average number of stars, the total number of stars can be estimated.
It's also important to note that these estimations are based on current technology and knowledge, and as technology and understanding of the universe improves, these numbers may change.