In my current shop, we manage close to 400 os images, about 200+ servers, > .5 Petabytes of Storage, > 20TB of backups/day and have only 5 FTEs and 2 contractors.
A lot of the FTE to Server/User ratio depends on how organized your shop is. If you have managed to automate most of your SA activities, it's not unusual for 3-4 FTEs to manage a 1000 server shop.
The key words are:
1) Standardization of platform (have at the most 2 or 3 platforms, chosen for specific roles)
2) Standardization of OS/Build (have 2 or 3 OSes you can manage well, eg: Solaris, Linux)
3) Standardized Architecture (define your architecture and stick to it -- SAN, TAN, etc)
4) Automation tools (such as a good monitoring solution, Fault Management system, etc)
5) A good analytics tool (such as splunk, which will simplify your life with automatic log indexing, parsing).
In an organization with centralized resources (trends have been gravitating towards centralized infrastructure, datacenter consolidation etc), this becomes easier. In a distributed IT shop it is harder.