Do you mean payment processors who processing credit card/check/phone payments on your sites or services such as paypal which you can also use to send and receive money to/from anyone using your balance?
No one will tell you exact number because many getting in the biz all the time and some are leaving. Here you will find many, but not even close to all of them: http://imerchantwizard.com/db/ If you want to find the ones that meet your specific requirements, launch the wizard on the main page: http://imerchantwizard.com