On average (as I do understand that servers go on and off the network, etc.) how many Tor exit nodes are there in service at any given time? (It is probably going to be an informed estimate.)
Check out "Torstat": http://torstatus.blutmagie.de/ The table at the bottom of the page (Aggregate Network Statistic Summary) shows you the current number of exit nodes.
This is now graphed by the Torproject at https://metrics.torproject.org/relayflags.html (826 exit nodes today).