There's not any way to know that right now, because viewing a post is purely a local action taken on each user's computer, and it isn't recorded in the blockchain. Theoretically, each app could measure it individually and the resulting data could be aggregated, but it would require cooperation of all the various apps that allow you to read hive posts. You would probably also want to discard "multiple" hits by the same account, which further complicates the task.