[{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([{$project:{total_employees:{$reduce:{input:"$departments",initialValue:0,in:{$add:["$$value",{$size:"$$this.employees"}]}}}}}])