As many as you like - if you haven't added any new hardware.
If it asks for reactivation just ring up MS and get them to do it (or do it via the net) - just say that you've added a new motherboard and its popped up asking for activation. Sound innocent. They might quiz you about peer-2-peer sharing and whether you use it to download movies and music and stuff like that - just make out you've never heard of!!