At least 350, 000 times, according to Fallible Blogma in 2010.
The same statistic is repeated on a few other sites. I'm not sure where it's from, how it was arrived at, or how accurate it is. But it's a very pleasing thought.
Has Mass ever been celebrated in outer space? It would appear not, from this post by Father Z.
I think I got to thinking about all by way of thinking about Ireland's much-reported annual biker Mass.