Database
[{"_id":ObjectId("5b3314a12b05b1b247366f48"),"email":"abc@gmail.com","qwerty":[{"id":"5ba4ebbad1b5eaf038841302","status":"inprogress","Date":"2018-08-20"},{"id":"5ba4ebbad1b5eaf038841303","status":"inprogress","Date":"20-08-2018"}]}]
Query
db.collection.aggregate([{$match:{"email":"abc@gmail.com"}},{$unwind:"$qwerty"},{$group:{_id:{$month:{$dateFromString:{dateString:"$qwerty.Date"}}},count:{$sum:1}}}])