Is the number 666 a superstition, and is a person's avoidance of writing it for fear of imitating disbelievers considered a superstition, and is there a supplication or intercession for expiating superstitions, and how does one repent from actions based on superstitions?

The belief that the number "666" is a number special to Satan or the Antichrist is a superstition originating from disbelief (kufr), and it is not permissible for a Muslim to hold such a belief in its particularity; rather, it is like any other number. This belief is considered an imitation of the disbelievers, and the Prophet, peace and blessings be upon him, said: "Whoever imitates a people is one of them." Repentance from these superstitions is by abandoning them, regretting them, resolving not to return to them, and believing that benefit and harm are solely in the hand of Allah, and that the Sharia aims to protect minds from superstitions.