I think typically ten or less, but I think the attrition is mostly kids deciding to opt out, rather than being booted. They get there and change there mind either immediately, or by the end of the day. Some just realize they are going to miss home, and quit.
I think the days of booting kids in large numbers based on strict compliance with the standards, unfortunately, may be history. My son was shocked and disappointed during Beast that a few conspicuous "slackers" were allowed to continue, despite falling behind continually, complaining, and failing to complete things everyone else did without complaint. He said also that the slackers came in every shape, color and gender, but that the vast majority, and almost all others, were impressive and outstanding. He left Beast, as he did when he finished his first year at West Point, loving the Corps of Cadets, and being a part of such a tremendous group. Frequently he raves about how impressive the men and women are.