11 that I actually sit in front of on a regular basis - mix of Pros, MBP's 17/15 and the Air + spare (which has been needed as soon as I picked it up)
3 x Pros @ home: General noodling. One Boot Camps Vista, but not all the time
4 x Pros @ work: Reviewing and modelling crunched data from the servers, related stuff
Air +spare @ everywhere: Mostly presentation - combined with notetaking, messageing and related stuff. Spare acquired recently, already in use while other one gets Applecared yet again

1 x 17" Macbook Pro @ home: More home noodling. The purchased spare is collecting dust so far, which has to be some kind of record
1 x 15" Macbook Pro @ work: Research collaboration, and also general office automation - everyone has one. We have plenty of spares to go around for the regular occasions when something goes wrong
Everything else is done under Windows.