Why is number 13 not used in Formula 1? Is it just superstition? Was the number ever used?
Number 13 isn't in use just from superstition.
The FIA Formula 1 World Championship entry list does not have a number 13. The number is generally considered to be an unlucky number. It was used in the early years of the sport but in the 1920s it was dropped after the Delage factory team suffered two fatal accidents with the number in the course of just a few months. The first was Paul Torchy, who crashed into a tree during the San Sebastian Grand Prix in September 1925 (below).
Basically, yes it's just superstition. I remember noticing this in the 1990 season where the Osella/Fondmetal team were only running one car (Olivier Grouillard). Logically they should have run car #13 but were allocated #14.
Here is one of a number of sources for the superstition story online.
Note that, after a rule change in 2014 which mandates drivers to make such a decision, Pastor Maldonado opted for the number 13 for the rest of his F1 career. As of 2021, Maldonado has not raced in F1 for more than two seasons, and so the number is available for any new entrant who wishes to use it.