integer llGetRegionAgentCount( );

Returns an integer that is the number of avatars in the region.


The value returned by this function is technically the average number of agents who were in the region for the past second, rounded to the nearest integer. This means that there is a slight (<1 second) delay in agent count when an agent enters or exits a region.


    touch_start(integer num_detected)
        integer numberOfAvatarsInSim = llGetRegionAgentCount();
        llSay(0, "There are currently "
            + (string) numberOfAvatarsInSim + " avatars in this sim.");


Credit to LSL Portal with changes for brevity and clarity.