The souls are infinite in number. I will give you a very simple example. We stay in one house for some time. After some time we shift to another house. Again, we go to another town or place and shift to another house.Thus, we are going from one house to another or from one place to another. Just like this, the soul also stays in one body for some time. At the death of the person, the soul leaves the body; then it either attains salvation and reaches Paramapada; or, it goes to Svarga -(heaven); or directly takes on a new body. Where the soul goes to Svarga, after experiencing pleasures the soul returns to earth and takes on another body, as a man, or an animal, or a bird or anything. Thus, the soul also goes from one body to another, just as we shift from one house to another.