Well theres over 150 listed here :
http://www.affiliateseeking.com/netwo/23000002/1.html
Personally I believe that's just a handful of them lol.
I've done work with CPA Networks and earned from a few of them, I've come accrossed loads and from say 2007/08, the rise in new networks opening up on the web has dramatically increased, unless it's just me noticing them far more.
I would say there is over 3,000 of them, not sure how many over. could be a few hundred, could be a good few thousand. Don't think theres less than that though. Some networks are good quality, some look like they have been set up over night, and then so are totally poor and not worth bothering with.
Hope this has answered your question.