[{"company":"abc","countries":{"australia":["banana","kiwi","avocado"],"germany":["apple","cucumber"],"columbia":["banana"]}},{"company":"d","countries":{"australia":["banana","kiwi","avocado"],"columbia":["banana"]}}]

db.collection.aggregate([{$set:{count:{$reduce:{input:{$objectToArray:"$countries"},initialValue:0,in:{$add:["$$value",{$size:"$$this.v"}]}}}}},{$sort:{count:-1}},{$unset:"count"}])