db={users:[{email:"abc",team:"team1"},{email:"abc2",team:"team1"}],teams:[{_id:"team1",name:"team1",active:true},{_id:"team2",name:"team2",active:true}]}

db.teams.aggregate([{"$lookup":{"from":"users","localField":"_id","foreignField":"team","as":"users"}},{"$addFields":{"user_amount":{"$size":"$users"}}},{"$unset":"users"}])