Is it common to have two birthmarks?

It's not uncommon - I have a brown birthmark on one of my buttocks, and a reddish mark on the back of my neck. I also have about 50 small moles on my body, although having appeared later in life these are not really "birthmarks". My wife also has two birthmarks - a purple patch at the base of her spine, and a faint "cafe au lait" mark on the jawline.

I have noticed that many other people have more than one birthmark, and most people have at least 10 moles on their bodies.