Memory requirement also called as Memory footprint for a Vuser can be called as a function that is dependant on Vuser type (Web, CITRIX, SAP GUI); application and system where you intend to run the test.
So if your Web/HTTP Vuser takes 5MB when running as a process, 50 Vusers would take 250 MB. Then remote desktop connection , third party program (that is continuously running in background) , memory consumed by application all summed will give you the memory requirement for a test.
You can calculate it empirically. Do it for say 10 Vusers and than extrapolate that requirement for x no of Vusers.