A few hundred, but only 40-some are actually in use at the moment. I've a few P4 Xeon system, only three single-P4 based systems, an assortment of single and dual Athlon machines, two quad-PIII Xeon systems, five dual-PIII Xeon systems, 20-some dual PIII machines, about 30 laptops, and the rest are mostly single PIII or celeron-based. (Oh, and two Cubix boxes filled with single-board computers.) I don't know if Sun machines count, but I have about 50 of those, including about a dozen Ultra 2 machines, one quad-450 Netra, five Cyclequad Compute Servers, and a few dozen lower-end machines (Ultra 10s, Ultra 5s, Ultra 1s, 20s, and one Sun laptop)
The 40 or so computers in use are on all the time: there's four DNS servers, five user login machines, two news servers, a few mail servers, several web servers, one dedicated ftp server, a firewall between the internal and external networks, a dedicated NFS server, etc.
As far as the ones only I use, I have two main workstations that I do most of my work on, but I still have to keep switching back and forth between the new video editing system and the older one; then there are the media server and DVD duplication machines, which I was using constantly for the last several weeks, but I haven't fired them up for a couple of days now.
I've got an assortment of laptops used as network and system status monitors, and those are on all the time, and other laptops scattered around the house used mainly for casual browsing and bedtime reading.
Then I've got a collection of servers-to-be that aren't fully configured and tested, and some of those are running all the time, but they're not really being "used," since they're just being tested.