well...
If you count work computers I have had 100s if not more. Everytime they would get a new shipment of desktops or laptops in, I would grab one for myself so "I could learn the new product" we were deploying to our users. So, I basically got a new desktop every 6 months, and I have been in my field for almost a decade. I also have been supporting both PCs and Macs for that whole decade so I have always had both on my desk.
Right now in my office I have a G5 dual 2.5, an iMac C2D, macbook, macbook pro and a mac mini, well 2 minis technically but one is used to automate a certain task.
I have used windows since 3.11 and even had NT4.0 on a dual pentium pro system, all the way up to vista, including server OSes from server 2000, adv server 2000, server 2k3.
Mac OS since OS 7ish and all the way up to 10.5.
Linux since 1999ish which was about SuSe version 5 or 6, now they are up to 10.2 I think.
I also did at one time have a Unix machine as well as Qnix and BeOS as well, but never really used them as much as the others. At my first tech job we had this system in the corner all of us just tossed OSes on at random to try them out.