Anent Steve-the-Geek's comments: if Windexen (3.0, 3.1, 95, 98, NT, 2000, Me, XP, Vista, ...) only count as one, then z/OS and its predecessors (OS/360, PCP, MFT, MVT, OS/VS, VS/1, VS/2, SVS, MVS, ...) only counts as one. CICS was many things, but it wasn't an OS. And, along with OS/360, there was DOS/360 (and its progeny: DOS/VS, VSE). Before all of them, there was IBSYS.
Early Cray's ran NOS and SCOPE.
Who can forget TOPS?
And a list of OS's without including Multics? For shame!
How Many Operating Systems Can You Name?