Actually that naming convention is misleading because if we go past 100 gmail just starts a new conversation and if we don't specifically change the number it just stays the same. We're currently 24 emails into our third "#49", for example. A quick text-grab, regex, and spreadsheet-sum later gives me a total of 5787.
That convention started June 21st, 2011.
This tells me that's 646 days.
So, on average, 9 emails a day, only counting the "standard" threads. But the actual density varies a lot from day to day.
I'm also the jerk who points out the inconvenient statistical things in the group conversations, in case that wasn't clear. Someone else handles the more convenient statistical things.