Database
[{catId:101,chapter:[{id:"1234",title:"Title 1",code:"X12.3"},{id:"abcd",title:"Title 2",code:"X12.4"}]},{catId:102,chapter:[{id:"abcd",title:"Title 2",code:"X12.4"}]}]
Query
db.collection.aggregate([{$unwind:"$chapter"},{$group:{_id:"$chapter.code",count:{$sum:1}}}])