For races in the US in 2010, about 3000 was the limit. But the definition of 'Sold Out' has increased remarkably over the years (with the exception of 2008). And Canada's on its way to 3400 for 2011.
As of January 2011: Arizona, Florida, Wisconsin, Coeur d'Alene, and Lake Placid are sold out to general and foundation entry; Canada is near 3300 total entrants and still offering foundation slots (121 as of 1/27/11). After tough races in 2010, triathletes are not flocking to register for Louisville and St. George; high DNF rates are partly to blame.
Many of the most popular Ironman races in North America are deemed 'Sold Out' the same day registration opens. How many entrants can register for 'General Entry'? How many additional Foundation Entries? The numbers aren't made public, but we've done the analysis. Our estimate: roughly 2700 General, 300 Foundation Entries, around 3000 total. Canada is the exception for 2011, so far.
Combined General + Foundation Entries seem to increase by 100 to 200 athletes each year. In 2005, races were Sold Out with about 2200 to 2400 entrants. By 2010, up to 3000 total entries in a race seemed to be the limit for most races.