I'm pretty sure there are other threads on this, but it really depends on what your users are going to run on it.
There are some official figures in documentation on Citrix's site, but remember that Microsoft originally recommended a 386 with 4Mb RAM for Windows 95...
If it's just lightweight MSOffice stuff - Word, Excel, then a single proc server should support 40 users comfortably. As soon as you start thinking about using bespoke applications or databases, financial packages, etc you enter into unknown territory.
The way to find out, of course, is to pilot a few users and establish their needs by benchmarking the server as they go about their normal work. If you can identify "power" users early on, then you will get a better picture of the amount of resources you will need.
If the users are remote, then consider the bandwidth that they will necessarily have to share.
I hope this helps CitrixEngineer@yahoo.co.uk