Posted this in the "In at Columbia" thread, but now that I think of it, it has more relevance to this one:
If anyone's interested, I did a pretty quick analysis of LSN to see how many Hamiltons and Butlers went out every year.
2010 cycle: 14 Hamiltons 8 Butlers
2009 cycle: 14 Hamiltons 17 Butlers
2008 cycle: 12 Hamiltons 17 Butlers
2007 cycle: 24 Hamiltons 4 Butlers
2006 cycle: 11 Hamiltons 8 Butlers
Based on this, you could tentatively conclude that there's not many more Hamiltons going out, but there are a bunch of Butlers still waiting. It seems like they deliberately cut Butlers in favor of more Hamiltons during the 06-07 cycle, then decided to go back to the old way of doing things.
Based on this information, and random anecdotes from other threads, my guess is that 50-70 Hamiltons are awarded every year, and that Butlers are only slightly more common.