The total amount of RAM needed depends not only on the number of concurrent users, but on the size of your data model (number of fields, number of distinct values, size of values etc.). How big is your .qvw file?
Then also your front end might add the need for a lot of RAM (Charts, expressions (e.g. using aggr() function). Can you tell us something about that?
And then each concurrent user will add on top a per-user-footprint (very roughly some 10 percent of the application RAM footprint).
So it is hard to say what your RAM requirements are only from knowing the number of users.