Computers (or laptops) from other people, libraries and other sources -- basically whichever ones that aren't yours -- don't count. If you happen to have two or more at any time, just count one of them (since it would only require one of them to access MSF). Definitely count any computer (or laptop) that has come and gone.
I'm on my fifth computer since I've joined MSF, so it's four for me. While my last computer is still here and operational, I no longer use it for the Internet and intend to hook up its hard drive to my new computer one of these days.