Mongodb query for getting count of chapters in collection.

Nested array aggregation query.

db.getCollection('your collection').aggregate([
  {
    "$project": {
      chapters: "$books.chapters", _id: "$_id"
    }
  },
  {
    "$unwind": "$chapters"
  },
  {
    "$group": {
      _id: null, totalChep: {
        "$sum": {
          "$size": "$chapters"
        }
      }
    }
  }
]) 

//Data model.
{
  _id: 01,
  books: [
    {
      name: 'example 1'
      chapters: [{chap: 1}, {chap: 2}, {chap: 3}]
    },
     {
      name: 'example 2'
      chapters: [{chap: n}, {chap: n}]
    }
  ]
}