The number is probably around 100% if you monitor a person (notice I didn't say patient) long enough. The thing is for some reason only some people feel them, we are the unlucky ones who do. On the plus side all of our worrying results in a good cardiac workup which can give us some peace of mind.
Everyone has extra beats, some people have more than others. If you read back in the post, you will find that most deal with extra heart beats.