According to this article the Secret Service had about 225 agents at the time. Currently the Secret Service has 3,200 special agents according to the Secret Service FAQ.
SECRETS OF THE HUSH-HUSH BOYS
Since 1865, for your welfare and his own, the U. S. Secret Service man has been a very hard guy to get to know.
By Glenn D. Kittler
WITHIN 24 hours after the attempt on President Truman’s life in 1950, every newspaper in the world carried the story but not one account named the men whose bullets had riddled the assassins. The reason: U.S. Secret Service agents have a passion for anonymity. They are never identified; they are never even photographed, except when one of them near the President accidentally gets into a camera’s view.