Database
[{user_number:12,region:"Pune"},{user_number:13,region:"Mumbai"},{user_number:14,region:"Mumbai"},{user_number:15,region:"Punjab"},{user_number:16,region:"Delhi"},{user_number:17,region:"Pune"}]
Query
db.collection.aggregate([{"$match":{user_number:{$in:[12,13,14,15]}}},{$group:{_id:"$region",count:{$sum:1}}}])