Complex sentence: In general a manifold may have no volume form or infinitely many volume forms, however many classes of manifolds come with canonical volume forms, that is, they have extra structure which allow the choice of a preferred volume form.
Split into simpler sentences: In general a manifold may have no volume form, or infinitely many volume forms. Many classes of manifolds do come with canonical volume forms, that is, they have extra structure which allows the choice of a preferred volume form.