[{name:"ABC Bank",hospitalList:[{city:"Delhi",hspName:"EEE hospital"},{city:"Delhi",hspName:"FFF hospital"},{city:"Surat",hspName:"MMM hospital"},{city:"Noida",hspName:"GGG hospital"},{city:"Surat",hspName:"HHH hospital"},{city:"Surat",hspName:"NNN hospital"},{city:"Surat",hspName:"PPP hospital"},]}]

db.collection.aggregate([{$match:{name:"ABC Bank"}},{"$unwind":"$hospitalList"},{$group:{_id:"$hospitalList.city",count:{$sum:1}}},{$addFields:{city:{$concat:["$_id"," - ",{"$toString":"$count"}]}}},{$project:{_id:0,city:"$city"}}])