So I read an article today that there are currently 6 former women's US Open champions slated to play this year at the Open. The list is Sharapova, V. Williams, S. Williams, Henin, Kuznetsova, and Hingis. This seemed like a lot, so I investigated.
I only went back to the early 80s, since the data is harder to read through prior to that. Also, I am only considering past champions in the draw, not future champions. And I'm only considering singles here, so although Navratilova entered mixed doubles last year, she does not count.
On the men's side, the most in a given year was 6 in 1992, which has Sampras, Lendl, Becker, Edberg, McEnroe and Conners. This year should be 4 assuming Federer, Roddick, Safin and Hewitt do not get hurt. The fewest in a year was 2 in 2003 (Agassi and Hewitt) and 1999 (Agassi and Rafter)
On the women's side, 6 would set the record (at least for recent years). I found 5 instances with 5 former champs (2006, 2005, 2002, 2001, 2000). In 2003, there was only 1 former champion, Lindsay Davenport. This is of course just a freak occurrence, because Hingis had "retired" and if I recall correctly, both Williams sisters were hurt.
I'll replicate this for the other grand slams when I get a chance.