[{"status":true,"data":[{"_id":"61a2a9680b122bc154cbd6af","ownerName":"Ajay","mobile":878787878,"designation":"IT","gender":"Male","age":26,"carDetails":[{"carName":"BUY","color":"blue","sheets":8,"avgSpeed":105,"price":850000,"model":"C110"},{"carName":"GTR","color":"blue","sheets":4,"avgSpeed":105,"price":98000,"model":"G120"}]}]}]

db.collection.aggregate([{"$addFields":{"data":{"$arrayElemAt":["$data",0]}}},{"$project":{"status":1,"ownerDetails":{"_id":"$data._id","ownerName":"$data.ownerName","mobile":"$data.mobile","designation":"$data.designation","gender":"$data.gender","age":"$data.age","total_car":{"$size":"$data.carDetails"}}}}])