[{"name":"John Doe","City":"OK","Prepaid":"Y","Ethnicity":"a","Zip":"06516"},{"name":"Jane Doe","City":"CA","Prepaid":"N","Ethnicity":"b","Zip":"12321"},{"name":"Jule Doe","City":"OK","Prepaid":"N","Ethnicity":"a","Zip":"06516"},{"name":"Jake Doe","City":"OK","Prepaid":"Y","Ethnicity":"a","zip":"06516"}]

db.collection.aggregate([{$group:{_id:"$City",Y:{$sum:{$cond:[{$eq:["$Prepaid","Y"]},1,0]}},N:{$sum:{$cond:[{$eq:["$Prepaid","N"]},1,0]}}}},{$project:{_id:0,"City":"$_id",Count:{"filter":"Prepaid",count:{Y:"$Y",N:"$N"}}}}])