[{_id:1,"Home":"A","Away":"B"},{_id:2,"Home":"B","Away":"C"},{_id:3,"Home":"C","Away":"A"},{_id:4,"Home":"C","Away":"B"},{_id:5,"Home":"A","Away":"C"}]

db.collection.aggregate([{"$set":{"obj":{"Home":"$Home","Away":"$Away"}}},{"$project":{"Away":0,"Home":0}},{"$set":{"obj":{"$objectToArray":"$obj"}}},{"$unwind":"$obj"},{"$group":{"_id":"$obj.v","count":{"$sum":1}}}])