The main physical factor that governs the scalability of Citrix servers is the RAM. The majority of the operations are performed by three services: zenworksWindowsService, ZenNotifyIcon, and zenUserDaemon. The RAM consumption depends on the number of sessions and the number of effective assignments for each session.
For the minimum hardware recommendations, refer to the Citrix website. If you can provide hardware that exceeds these recommendations, your system will perform better. Additional processing power and faster drives can make the systems more responsive.
The other factors that you need to consider include the following:
Device refresh frequency
Bundle schedules
System requirements