When you look in the Archived Challenges sections, it says "To date, 993 challenges have been archived." with the last one being "Action Shot III".
That would mean:
Free Study 2009-02 = Challenge #994
Light on White IV = Challenge #995
Self Portrait Without People = Challenge #996
Skin = Challenge #997
100 Years Old = Challenge #998
Hidden Gem -- 1,000th Challenge! = Challenge #999
Fun with numbers and superstitions. Challenge #666 never occurred, for obvious reasons. #999 upside down. And, as far as ID numbers go, there must have been some other numbers skipped for various reasons.
