Over what time period? We have an average of less than 1 exception per month.

I have also worked on a larger web app that would get thousands of exceptions per day.

I think near 0 is very possible as long as you aggressively attack all exceptions as they show up. Once you start letting some go they tend to pile up exponentially. I keep my phone on me and every time, day or night, that an exception occurs I get notified; it keeps me very motivated to fix problems quickly.