TL;DR. My apologies.

To answer the OP for my part.
"How many houserules does 5E need?"
None, or as many as you feel appropriate for your campaign.

While you can play the game explicitly by the book, there are only so many styles you can actually play with the rules as-is. If or when I have an idea for a campaign that isn't covered by the rules already, then I prefer making adjustments or outright homebrewed rules. It's not that the edition itself would need any houserules, it's the campaign played that might need some. That's my opinion at least. And I'd do that for any system used, not just D&D, if necessary.