Four: Lincoln, Garfield, McKinley, and Kennedy. Attempts were made on Teddy Roosevelt, Franklin Roosevelt, Harry Truman, Gerald Ford, and Ronald Reagan. (I was at the hospital when the brought Reagan in...talk about pandemonium, that was a week I will never forget.)
Lincoln was shot by John Wilkes Boothe, an embittered Secessionist. Garfield was technically killed by Charles Guiteau, a disappointed office-seeker; but in reality he died of the treatment he received from ignorant doctors who didn't know that in "probing for the bullet" they were exposing him to peritonitis, which is what eventually killed him. McKinley was shot by Leon Czogolsz (I have probably misspelled the name), who walked up to him "with a handkerchief over his hand" as the song has it, in a train station. And Kennedy was shot by Lee Harvey Oswald, AND NO ONE ELSE, no matter what anyone tells you.
If you can find a copy of a book entitled "Fired In Anger," by Robert Elman, it has the stories of these and many other famous and infamous shooters and shootees in it. Well worth reading.