You really can't do any better than some back-of-an-envelope guesswork for this sort of thing. How much memory will it take to process 10000 users? Measure it for 1000 and multiply by 10.

Of course the best thing to do is just test it. Generate test input for 10000 users and see if it breaks. Keep adding more until it does. Then change your algorithm to use less memory if you need it to go further. Remember that you can almost always trade speed for memory.