The answer to your question is -- "It all depends...!"
It largely depends on how the application was written and how well your network works.
If the application were not written well and/or it required numerous uses of data tables EXCLUSIVE or it used FLOCK() many times, then you are likely to quickly run into user count problems as separate users run into problems attempting to access data tables.
Or if the application were not written in a 'forgiving' manner, such as not using re-try methodologies on error handling, etc. then the users might possibly begin to encounter problems as the user count increased.
If your network was not totally stable, then the time to acquire data from the data tables might grow to be excessive.
For a well written application with a good performing network, I shouldn't be surprised to see over 100 users (maybe multiple 100's of users) concurrently using the VFP application with no problem.
Good Luck
