What is the maximum number of cores used by a single MySQL server in a production setting?
The MySQL server can use any  number of cores. But remember that a single connection/query will only use one thread. So while MySQL supports many core the queries are processed on one core each.
-  wrt. recent technology.
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.