On a regular basis, just a couple of flavours of Linux (I have Gentoo on my own systems, I maintain Fedora on a few relatives' systems, and I have a few embedded mostly non-GNU Linux devices that I fix with BusyBox when necessary), OpenBSD (on an old box, running a few services on my home network, e.g. CUPS, NTP, SMB), and Windows 7 (on its own HDD on my desktop, only used for a couple of games).
I count that as three.
I guess if I include things like phones, consoles, etc, I could add a few more to that, but it only counts for me if I'm actually getting stuck into the OS proper and not just using the weak end-user GUI as intended. I count all Linux distros together as well, as they're usually pretty much the same thing with a couple of variations in tools/services/versions and package management.