In The Vampire Diaries Season 4, first it was twelve council member who were sacrificed, then we know in flashback a witch tricked to sacrifice twelve humans to break the Sire Bond and even twelve hybrids were need to be sacrificed for Expression magic. But why only twelve and not any other number? Is there any in/out of universe explanation for it?
Note: Seen upto season 4.
Per the comment above, there isn't an in-universe explanation for the choice of 12 as a number.
That said, 12 is a highly significant number in various religions, esp. the typical christian look a likes which show up in vampire lore. As a result, it's a number which audience members are likely to accept as significant without needing much of an explanation.
Not sure why the writers have chosen the number twelve but from the universe, it'll turns out that those events (except for breaking the sire bond, but it was an Expression magic too, so the same rules applied) (SPOILERS) were orchestrated by Atticus Shane to complete The Expression Triangle and it had to be 3 massacres sacrificing 12 person in each and he made sure that 12 humans/hybrids/witches were sacrificed in each massacre.
- An Expression Triangle is the combination of three brutal massacres that happen at separate equidistant locations. These massacres mark the Earth with power and can be used as focus points for witches who practice Expression.
and from here
Unbeknownst to Bonnie, she was drawing on the power of two separate sacrifices of twelve that were orchestrated by Shane behind the scenes in her practice of Expression magic; the first was a sacrifice of the twelve humans who were blown up in the explosion at Pastor Young's ranch, and the other was a sacrifice of twelve unsired hybrids
...managed to manipulate Bonnie to help him complete the Expression Triangle by killing twelve witches
