Megan,
I have been unable to locate an exact "count" of the frescoes in the Villa of Mysteries; however, I did find a web source (Catholic Education) with a map of the Villa and it does indicate there are other frescoes. Here is a link to that site:
 http://class.lism.catholic.edu.au/ahist-dvd/ahist-hsc/Pompeii/nature%20of%20sources%20and%20evidence/private%20buildings/villas/villa%20of%20the%20mysteries/villa%20of%20mysteries%20plan.jpg