MySQL: What is the maximum number of cores used by a single MySQL server?
I recommend you MySQL 5.5 -- add this manual page and licence page and this benchmark together and you will see why. To clarify further: MySQL -- rather, the InnoDB storage engine -- will use all cores in the system but up until 5.4 it actually slowed down with a higher number of cores, Google fixed this and the first MySQL to contain it is 5.5.

This fix also can be found in the 1.0.4 plugin and newer which can be enabled in MySQL 5.1.38.