Database
[{type:"COUNTRY",users:[1,2],name:"SG"},{type:"COUNTRY",users:[3],name:"SG"},{type:"COUNTRY",users:[4],name:"IN"}]
Query
db.collection.aggregate([{$match:{type:"COUNTRY"}},{"$unwind":"$users"},{"$facet":{"groupCountByCountry":[{"$sortByCount":"$name"}],"totalCount":[{"$group":{"_id":null,"count":{"$sum":1}}}]}},{"$project":{array:{"$concatArrays":["$groupCountByCountry","$totalCount"]}}},{"$unwind":"$array"},{"$replaceRoot":{"newRoot":"$$ROOT.array"}}])