I own three computers at this time, but work primarily off my Windows 8.1 Hewlett-Packard.
The HP has the most recent software on it, for example, Office 2013 (I can't stand Office 365, I don't know about you, feel free to sound off on that), SketchUp 2015, and has the latest (but not necessarily greatest) components onboard.
I also own a Dell Latitude E6400 that runs Windows 7 SP1. I have a couple of programs that just don't work under Windows 8.1, even with compatibility layers, special switches, or running from the DOS Prompt, and that's why I retain it. Finally, I have an experimental unit that is a Dell Optiplex GX280 (or something like that) running Windows 2000 SP4. I only use it when I'm feeling nostalgic, and that's not often.
Additionally, I run various historic versions of Windows on either VirtualBox or Virtual PC 2007 from my HP. I don't run them at the same time, though- I've tried and have run my computer aground more than once doing just that, so I don't.
Back to the question at hand: is it okay/normal to own more than one computer? Here's my take on that.
Normal: Maybe...
Acceptable: Certainly, so long as you don't trip a breaker in the process.