For 20 concurrent users, you might want to have atleast 4 rbs. 2 Gb should be fine. Make sure that there are no file size limitations on the OS if you have Autoextend ON for the ts. You may want to consider using optimal storage parameters for the rbs (if they are not). You may want to (re)size the rbs based on max(rssize). This is a good way to size them and monitor their behavior for future modifications.