[{"userId":1,"orders":[{id:1},{id:2},{id:3},]},{"userId":2,"orders":[{id:4},{id:5},]},{"userId":3,"orders":[{id:6},]},{"userId":3},]

db.collection.aggregate([{$match:{orders:{$exists:true}}},{$group:{_id:null,totalCount:{$sum:{$size:"$orders"}}}}])