I have been looking around for a while now, but I can't see anywhere how many audits I have failed.
Is this purposely not being exposed to users?
Where can I see some history of the audits I have failed?
Is there a raw number per category?
I don't know anywhere in the UI where this is exposed as a raw number or percentage.
As far as I know, you have to figure it out yourself. You can do that by looking at your recent activity in your profile. Filter by "reviews", and click through the items looking for review audits. Whether you passed or failed is clearly indicated at the top of the results page.