There is an unbylined piece posted by the R&A analyzing the 339 rules incidents handled by the "international team of referees" at the 2012 Open Championship, with a numerical breakdown of the Rules situations that arose.
I'm not sure why I found this surprising considering the severity of the rough, but it did sound like a lot of unplayables:
One of the most commonly used Rules in golf is the unplayable ball Rule (Rule 28). In total, 32 unplayable ball rulings were dealt with at the Championship, including Phil Mickelson’s drop after he found the deep rough above a bunker on the 8th hole. Click here to read more.
The weather, of course, can have a big impact on the Rules and 2012 was no exception. Due to the unusually wet weather that Britain has experienced this summer and, in particular, following a heavy rainfall on the Thursday night of the Championship, no less than 58 casual water rulings were overseen by referees. Of these, 37 related to casual water in a bunker where Rule 25-1b(ii) was applied. Click here to read more.