The truth is that there is no benchmark for this. I am a consultant and tend to take a sysadmin role for clients.
If you use Active Directory and store user files on the network then you can do stock images for each model of machine and a broken unit is a 20minute re-image (or swap fresh machine in from the pool) and your up and going, documents and all. This is where a directory services' up-front costs become justified.
With Active Directory I can manage machines and users very efficiently, keep user's files safe with shadow copies, backups, etc, and deploy software and printers to users easily. For linux to Active directory look at likeise-open or centrify for single sign on with the latery able to do group policy on linux machines.
I have 4 techs and myself. between the 5 of us we handle about 2500 or so users across our clients. Our clients that have been with us for a year or two are all have some sort of AD setup and have a much lower computer expense than before. oddly enough, newer clients account for larger expenses because they havent standardized their computers ad require more trips to their site and more billable hours.
I would imagine that if I had only established users, with computers on AD then my crew could handle 3000-3500 users without much overtime. If we did no managed computers then I think that 250 users per tech would be pushing it.
If you just compare those numbers, 600 vs 250 you can pretty easily see the cost savings for a managed network, either through AD, network, or other LDAP. a 1200 user network could be reasonably run by 2 IT guys vs really needing 4 or 5 to do the same job otherwise. let be conservative and say 4 guys at $40,000 each vs 2 guys at $50,000 (higher skills for 2 techs vs 4) and you see a $60,000 gap, which is much more than the CALS and servers needed for 1200 users. 1200 users is still in the 2 ADDS servers arena. lets say $3500 per licensed server and $35 per CAL and you save money on year 1, next 4 years are free!.
Right now it is kind of handy because my guys work their ~40 hours doing stuff on managed networks and pull 'overtime' going to customers sites or doing old computer triage and repair and get paid part of the service fee.
If you are at 600+ users per tech, then you really should be on some sort of directory service like AD. If you are not, I suspect you are spending a lot of your labor dollars spinning tires and not helping clients/users very well. That equals more compaints, less praise, and likely a lack of raise or bonus.