Hi there. Thats is a good question because it is one about names really. You see there is only one "Sun" - our Sun. This is the name we have given to our local star. However, there have been billions of other stars discovered. So, if we rephrase the question to how many stars have scientists discovered, then the answer is billions. If you take a look at this picture http://upload.wikimedia.org/wikipedia/commons/thumb/6/62/Starsinthesky.jpg/300px-Starsinthesky.jpg every single bright speck is a star which could be someone other civilization's "Sun".