Shadowing (sorry, I borrowed the term from Citrix) is running the user's desktop on your (the admins) desktop, and being able to watch everything they do - and even take control of the mouse and keyboard if you need to. It's incredibly powerful.
It's not really defined anywhere how many users can use T/S, because it depends on too many things;
1) How powerful the server is.
2) How many applications each user runs.
3) The type of applications being run.
4) Other things that you hadn't thought of.
The only way to tell is to run careful benchmarks for pilot groups of users before immplementation.
There are of course "rules of thumb", as in anything else;
If all users are only running MSWord or Excel, then you need about 12-20Mb RAM per user, PLUS 128Mb for the Operating system.
You can expect to get about 30 of the above type users on a single processor machine, 60 users on a dual-proc and 100 users on a quad-proc machine, so dual-proc is generally more cost-effective. This comes to about 1.5Gb RAM on a dual-proc machine for light use.
I normally spec 50 users per server, 2Gb RAM, 2x whatever processor gives best "bang for buck". For most organisations this works well. However, you need to be aware that app usage raises the bar, and thus need to get familiar with Perfmon fairly quickly!
Hope this helps
CitrixEngineer@yahoo.co.uk