The general rule of thumb for SAPGUI load tests is that you can run 50 virtual users on a 1 GB RAM and 1 GHz computer.
It is possible to execute more than 50 users on a computer if you have more power and more RAM, but the increase is not linear nor can this rule be applied to all operating systems.
The real bottleneck for SAPGUI testing is neither RAM nor CPU; it is the number of system-wide GUI resources.
The exact number of possible virtual users per computer is difficult to estimate; it depends on the scripts that are executed. If the virtual users execute transactions that contain lots of controls on the screen, it is more likely that the system runs out of GUI resources faster than with another script.
Therefore you should do some test runs to determine the exact number of users that you can simulate. Do this by executing an increasing workload. When the limit is reached you will see users that are either: