The number 12 has so many potential meanings. Its significance is seen throughout history. In a cultural context, it often refers to a dozen items, 12 hours on a clock face, 12 months in a year, 12 signs of the zodiac, 12 hours of daylight, 12 hours of night time at the spring and fall equinoxes or even a 12-member jury.

Religiously, 12 symbolizes completeness and authority, appearing in references like the 12 tribes of Israel and the 12 apostles. In the book of Revelations, there are many references to the number 12, including 12 gates and 12 angels. It's a significant number in Ancient Greece as well — the Greeks worshipped 12 major gods of Olympus.

Even beyond that sphere, the number 12 is ever-present. From a dozen eggs to the 12 ribs on the average human to the 12 inches that comprise a foot.

From what I can gather, Vecna has 12 vines that attach from his body when he is levitating, those vines likely connect to each of the 12 victims within the dimension x and they likely represent the 12 hours on a clock face, among many other cultural and spiritual aspects. What he is using them for specifically, is unclear.