I would have to say 5 and possibly 7.
JFK - Killed
Johnson - Murderer
Nixon - promised 1968 if he didn't run in 1964.
Ford - Coverup
George H W Bush - Outside the SBD building when they went in to get Oswald.
I would also throw in Truman and Eisenhower. When they both heard about the assassination my guess is they knew it was LBJ immediately.