A 12th level intellect is a term that was used to describe some of the smartest people in the universe. Metron claimed that only a 12th level intellect could have the slightest hope of solving the Anti-Life Equation and survive.
In the DC Comics, the Coluans (the alien race Brainiac belongs to) use a numerical level intellect system to rate intelligence. Usually, the average Coluan rates an Eighth level intellect, while the whole of humanity rates a first level intellect. However, Brainiac's family line, the Dox lineage, has always displayed above average intelligence, ranking at a twelfth level intellect. One edition of Superman Adventures had Brainiac establish cannons that would activate upon sensing Kryptonian DNA. Brainiac boasts of this, only for Superman to fly to the opposite side of Brainiac remarking that such cannons were a "...rather stupid concept for a 12th-level intellect!" as Brainiac gets hit by them full-blast.