db={"users":[{"_id":"4775222e-8e4f-4f84-8dba-b097291bbd39","surname":"Smith","forename":"John"}],"jobs":[{"_id":"f4bdda3e-0e8d-4e8d-b070-7d01421f5a51","description":"do something","manager":{"$ref":"users","$id":"4775222e-8e4f-4f84-8dba-b097291bbd39"}}]}

db.users.aggregate([{"$lookup":{"from":"jobs","localField":"_id","foreignField":"manager.$id","as":"jobsCount"}},{"$addFields":{"jobsCount":{$size:"$jobsCount"}}}])