There is a number of factors that can determine how many users you can have per server, including 32 bit vs. 64 bit, applications, is this all on a LAN or is it a WAN, are you going to virtualize, etc.
As a general rule I have found that if you are running say Microsoft Office apps and a database, about 40-50 users is about maximum.
You can get some more info in these articles:
http://support.citrix.com/servlet/KbServlet/download/14728-102-17413/Scalability%20-%20Number%20of%20Users%20per%20Presentation%20Server.pdfAnother forum discussing this:
http://www.brianmadden.com/forums/t/32214.aspxA VMWare paper here:
http://www.vmware.com/pdf/esx2_citrix_planning.pdfAnd if you are new to citrix Carl Webster from here at EE has set up a site with some good step by steps.
www.carlwebster.com