I don’t know of any chart per se for determining how many adimins are needed per x employees. We have one person providing desktop support to my division which has about 80 employees. In addition, this admin is also responsible for several other application servers.
When I was dedicated to desktop support, I was responsible for 106 Win 95b desktop machines and three servers (1 Unix, 1 NT and 1 VAX).
There is a case study on Microsoft’s site showing lots of information about a particular company, and I think they are a bit overloaded with Admins., but that may be a result of Active Directory.