It is difficult to quantify how many concurrent users can be supported due to a variety of factors that ultimately effect performance. According to the most common architecture samples, SPLMS can support from 50 to 3000 concurrent users.
SharePointLMS ability to support a desired amount of concurrent users is as much dependant upon its architecture design as it is on the environment it is hosted within and content being delivered to the end user.
From an architecture standpoint, Elearningforce International developed SharePointLMS based on a four tier model with an underlying database schema heavily reliant upon an extensive set of keys, indexes, stored procedures and views.
The hosting environment should provide ample bandwidth and properly configured servers utilizing load balancing and server clustering where appropriate.
Content developers should design courseware where average page sizes are small enough to service a large enough audience given the amount of available bandwidth.