Database
db={"articles":[{title:"Demo",tags:[1,2,3]},{title:"Demo1",tags:[2,3]},{title:"Demo2",tags:[1,3]}],"tags":[{_id:1,title:"JS"},{_id:2,title:"ReactJS"},{_id:3,title:"NodeJS"}]}
Query
db.tags.aggregate([{$lookup:{from:"articles",localField:"_id",foreignField:"tags",as:"articles"}},{$project:{_id:0,title:1,count:{$size:"$articles"}}}])