[{"_id":"0x026EFF","Stations":{"MP":false,"AS":true,"RW":true,"FT":true}},{"_id":"0x026F00","Stations":{"MP":null,"AS":true,"RW":true,"FT":false}},{"_id":"0x026F01","Stations":{"MP":null,"AS":true,"RW":false,"FT":null}},{"_id":"0x026F02","Stations":{"MP":null,"AS":null,"RW":true,"FT":false}},{"_id":"0x026F03","Stations":{"MP":null,"AS":true,"RW":null,"FT":false}}]

db.collection.aggregate([{"$group":{_id:null,MP:{$sum:{$cond:["$Stations.MP",1,0]}},AS:{$sum:{$cond:["$Stations.AS",1,0]}},RW:{$sum:{$cond:["$Stations.RW",1,0]}},FT:{$sum:{$cond:["$Stations.FT",1,0]}},}},{$project:{_id:0}}])