Statistically, as I calculated previously for another thread, around 28% of Pres. were VPs.
Most of them via death of their predecessor. if I recall it right 4 were elected to POTUS after serving as VPs(two being Adams and Jefferson), and one resignation.
there have been more VPs then POTUS though, and I haven’t calculated the amount of them that became POTUS.
9 is the answer!