It's 1 license per installation, so one for each operating system even if it's on the same physical machine. I quad-boot so should know ;-)
If in doubt Customer Service will explain it, they're a free phone call away as you know.
To speak with a Customer Service representative, call us at 866-622-3911.
Average contact length: 10 minutes
Estimated wait time: 5-10 minutes
Hours of Operation: Available daily, 6am-6pm Pacific
A hint: to avoid problems with the online account if uninstalling from a machine for whatever reason, try to use the normal method via Control Panel/Programs/Uninstall a Program (Add or Remove Programs in XP) first which will decrement the account automatically.
As that isn't always possible, a phone call to CS at the number above will sort it out. If they didn't do it last night try them again as only they can do it.
Extra licenses can be purchased if necessary.