I am going to have to go to PyCon. Not next year, as I'm going to be in Malta at the time, but hopefully in 2016. I don't know how many people attend, but the number is big, and they have sold old every year for the past three years.
I just got done watching one of the Keynote presentations online from PyCon 2014 held in Montreal April 9-17. I'm linking to it below, because it goes right to the heart of most of my academic work. But, it was especially exciting at the 20:05 mark. Where she highlights interactivepython.org and the Skulpt projects!