there are a multitude of factors that can influence this calculation.
some suggestions:
if you open your forms in tabs, avoid leaving too many tabs open, I limit it to a maximum of 4, when the user opens to 5, I remove it to 1,
avoid opening your queries with select * from table
open your forms without data, and offer a search feature, and limit the return on that search.
avoid using sequeced timer enabled.
avoid using forms, use frames instead.
avoid leaving the system open indefinitely, set a time limit to end the application without use.
my application has about 350 forms and consumes an average of 15 mb per user.
having more memory, and more processor, does not mean that you can add new users in the same proportion, you can reach the limit of the operating system, even if you do not use all available memory and processor.
if you have a large capacity server, it is better to create virtual machines on that server, than to use only a large server.