db={"profiles":[{name:"A",player_id:1},{name:"B",player_id:2},{name:"C",player_id:3}],"games":[{game_id:1,player_id:1},{name:2,player_id:2},{game_id:3,player_id:1},{name:4,player_id:2},{name:5,player_id:3}]}

db.profiles.aggregate([{"$match":{name:{$in:["A","B"]}}},{"$lookup":{"from":"games","localField":"player_id","foreignField":"player_id","pipeline":[{"$count":"cnt"}],"as":"player"}}])