[{"_id":ObjectId("5660c2a5b6fcba2d47baa2d9"),"customer_id":4,"customer_vehicles":{"cars":[{"id":1,"name":"abc"},{"id":2,"name":"xyz"}],"bikes":[{"id":1,"name":"pqr"},{"id":2,"name":"asdf"}]}}]

db.collection.aggregate([{"$project":{carsCount:{"$size":"$customer_vehicles.cars"},bikesCount:{"$size":"$customer_vehicles.bikes"}}}])