You can buy a single copy or a family pack for five computers.
http://www.amazon.com/Mac-10-4-Tiger-Family-Pack/dp/B0007LW1...
http://www.amazon.com/Apple-Mac-10-4-Tiger-VERSION/dp/B0002G...
There were no measures to technologically limit one's ability to perform multiple installations from a purchased OS X install disc. You could buy two different Tiger installers and they'd checksum the same, and there were no serial numbers to be entered. It was an honor thing.
I'm reasonably confident the OS didn't actively try to detect that it was running in a VM, but VMware Fusion (and presumably other such software) would prevent you from installing the OS.
There was a relatively simple work around (a hack) that allowed you to install in a VM. I think it was as simple as touching the right path.
Edited to add: You are correct, the technical restriction was on the VMWare side, but IIRC it was because of legal threats from Apple. Apparently earlier versions of VMWare would run SL without any problems, but the VMWare version I had would not not and I couldn't get a copy of the older version.
I greatly respected this trust and always bought the family pack for my Macs.
Yes, the licenses were per-user, if you as a person owned and used 20 macs, you only needed a single license.