Database
[{"Type":"Test","Items":[{"Name":"123","Id":"123"},{"Name":"123","Id":"123"}]},{"Type":"Test","Items":[{"Name":"123","Id":"123"},{"Name":"123","Id":"123"}]}]
Query
db.collection.aggregate([{$match:{Type:"Test"}},{"$addFields":{"Items":{$size:"$Items"}}},{$group:{_id:"Sum",Total:{$sum:"$Items"}}}])