Database
[{"CustomerId":13038,"AT":ISODate("2021-12-01T04:00:00.000Z"),"dwell":[7,6,12,6]},{"CustomerId":12036,"AT":ISODate("2021-12-01T04:00:00.000Z"),"dwell":[15,3,12]}]
Query
db.collection.aggregate([{$project:{_id:0,CustomerId:1,Count:{"$size":{"$filter":{"input":"$dwell","as":"num","cond":{$gt:["$$num",10]}}}}}}])