[{_id:"someId",studentName:"student1",subjectByCampusName:{campus1:["subject1","subject2"],campus2:["subject3"]},_class:"fqnOfTheEntity"}]

db.collection.aggregate([{"$addFields":{"countsByCampus":{"$arrayToObject":{"$map":{"input":{"$objectToArray":"$subjectByCampusName"},"in":{"$mergeObjects":["$$this",{v:{$size:"$$this.v"}}]}}}}}}])