[{"_id":{"$oid":"60509de7314e6812fe9fe5fc"},"dos":{"$date":"2021-02-16T00:00:00Z"},"createdAt":{"$date":"2021-02-16T12:00:39.444Z"},"updatedAt":{"$date":"2021-03-16T14:25:08.397Z"},},{"_id":{"$oid":"60509f43314e6812fe9fe68f"},"dos":{"$date":"2021-03-16T00:00:00Z"},"createdAt":{"$date":"2021-03-16T12:06:27.439Z"},"updatedAt":{"$date":"2021-03-16T12:06:34.879Z"},},{"_id":{"$oid":"6050d860b0a7ab6126a82763"},"dos":{"$date":"2021-01-17T00:00:00Z"},"createdAt":{"$date":"2021-01-16T16:10:08.495Z"},"updatedAt":{"$date":"2021-03-16T16:10:08.495Z"},}]

db.collection.aggregate([{$group:{_id:{$month:"$dos"},count:{$sum:1},},},{$project:{month:"$_id",count:true,_id:false,},},{$sort:{month:1,},},])