Most north American Ironman races sell out on the first day of registration. After that, several hundred enter sold out races via a Foundation Entry, which double the fee, with 50% going to Ironman Foundation charities. With all the demand to enter these races, it's remarkable that hundreds ultimately will never reach the starting line.
See Also
A Physiological Look at What the Body Goes Through in an IronmanOld school bondWhat the Initial Ironman IPO Filing Tells UsEverything You Need to Know About Bonds
Of those who do start a north American Ironman, about 95% finish. The exceptions in 2010 were Louisville and St. George, where several hundred triathletes did not reach the finish line.
