TWO~! The reason is on the both side of these two girls, there is a mirror. If you only have one, you can't see two girl's faces at the same time because they are facing to two different sides. But now you can. So there must be two mirror on the both sides of them.
Two~! On the picture you can see the faces of the two girl's at the same time. And there are exactly only two. You may tell this from their appearence. The reason caused this phenomenon is that there are two mirror on the both sides of the girls. With only one mirror, you will never see their faces at the same time. So there must be two.