> It sends me an email once a story hits a certain number of upvotes per minute

So do you get one email per-story that fits this criteria? Or is it some kind of roll-up?

Typically one email per story.

It checks every 5 minutes, and if more than one story happens to meet the criteria during that 5 minute bucket then it'll put them into one email (so the "hiring" checks appear in one email). But in reality because it's rare that 2 stories will trend within the same 5 minute bucket it ends up being one email per story.