[{username:123,rxm_number:5,shelf_number:12345},{username:123,rxm_number:5,shelf_number:12345},{username:123,rxm_number:5,shelf_number:12345},{username:123,rxm_number:6,shelf_number:12345}]

db.collection.aggregate([{"$group":{"_id":{"username":"$rxm_number","shelf_number":"$shelf_number"},"users":{"$sum":1}}}])