[{"assetId":"12356464q,34rtyurt,56546b","quantity":1},{"assetId":"84756e","quantity":5},{"assetId":"vcxsr,820dds","quantity":4}]

db.collection.aggregate([{$project:{quantity:{$multiply:[{$size:{$split:["$assetId",","]}},"$quantity"]}}},{$group:{_id:null,totalQuantity:{$sum:"$quantity"}}}])