When it comes to worldwide, entire internet using population, there is one ultimate source for browser stats. It doesn't list numbers, only a graph, but you easily see what browsers are how large compared to eachother, and their usage trend. I'm talking about the
Google Zeitgeist, of course. In cases other than that, there is no good estimation except for looking at your own logs, or possibly, but less dependable, having a look at the stats for sites with the same content type, update interval, target audience, and surrounding community.