{
 "_id" : ObjectId("59e05fa63e0f5015dcaeadbe"),
 "name" : "santu",
 "mes": "Test Message 1",
 "time" : ISODate("2017-10-13T06:39:34.273Z"),
 "replay" : [
 {
 "time" : ISODate("2017-10-13T06:39:55.365Z"),
 "_id" : ObjectId("59e05fbb3e0f5015dcaeadbf"),
 "replayer" : "santu",
 "replaymes": "Test Message Reply 1"
 }
 ],
 "__v" : 0
 }

The data structure is like this, check the total number of such acquisitions

db.Scores.aggregate([
 {$project: {'cnt': {$size: '$Scores'}}}
 ])

And write like this

db.mesModel.aggregate([
 {$project: {num: {$size: '$replay'}}}
 ])

The result is

[ { _id: 59e05fa63e0f5015dcaeadbe, replay: 1 },
 { _id: 59e063d85f465906606add9a, replay: 1 },
 { _id: 59e063da5f465906606add9b, replay: 1 },
 { _id: 59e063dd5f465906606add9c, replay: 1 } ]

I printed everything, but all I need is the total number of embedded data under a single data.

The final data I want is replays under Replay. My method is

const mes = req.body.mes
 db.mesModel.findOne({mes:mes},(err,doc)={
 console.log(doc.replay[key].replaymes)
 })