Instead of triangulation like that, which could get real complicated, you could also simply place enough in the areas you’re counting and limit the RSSI cut-off enough to measure those right next to it. That would work for specific areas you want to measure trends in, instead of general 3d spatial triangulation.

though some of the papers I read did this via measuring the time of flight of BT packets and how they were changed when people walked between them, and that really blew my mind!