A: It is possible for multiple users to be logged on to a single Windows 7 instance through the fast user switching feature, which lets a new person log on without logging off the current user. The previous user session runs in the background. Only one logon can be active at any time, but the other user sessions are all still running in the background.
There's no hard limit to the number of users that can be logged on through fast user switching, only the resource constraints of the machine. Every logon will consume a certain amount of resources, so eventually the machine will run out. If the users who're logged on have applications running when their sessions are switched, they'll consume more resources.