I would guess about 1000.
I've personally worked with DOS, MDOS, HDOS, PICK (a database OS), Linux, UNIX, Windows 286, 386, all the Windows versions, NT versions, MacOS, OSX, IOS, and the San Diego Supercomputer HPC System.
And there are a "host" of other ones!!!
FYI, You can run a number of these in VMWARE on your Windows 10 PC. Yeap!