How many Professional Thoroughbred riding Jockeys are there in the U.S.?

I have heard numbers from approximately 400-600. The actual number depends on several things. Jockeys have to have a license to compete and the number of licensed riders changes over time. Also, not every person who has a jockey's license actually rides in races, they might be an exercise rider who has a jock's license so they can ride in races three or four times a year. A lot of jockeys who ride regularly are members of the jockey's guild so a good idea of a more accurate number would be counting members of the guild, though there are plenty of riders who are not guild members.