[{org:"org",departments:[{name:"abc",employees:[{name:"number1"},{name:"number2"},{name:"number3"},]},{name:"mno",employees:[{name:"number4"},{name:"number5"}]},{name:"xyz",employees:[{name:"number6"},{name:"number7"}]}]}]

db.collection.aggregate([{"$project":{total_employees:{"$sum":{"$map":{"input":"$departments","in":{"$size":"$$this.employees"}}}}}}])