It's not a licensing issue
At current, Apple's highest core-count is 12. That's probably what you have.
The confusion may come from the CPU having Hyper-Threading, which is a technology that causes the operating system to see each physical core as two logical cores. In the case of your 12-core processor, the operating system would see 24 logical cores.
Technologies like Hyper-Threading tend to have situational advantages. If a program has a lot of wait times in it, such as if it's working with disks or network interfaces, then Hyper-Threading can allow the CPU to switch jobs more quickly, increasing performance. However number-crunching programs like Mathematica have relatively little need for this sort of juggling; in fact, it's often slower than not having Hyper-Threading at all. As a result, number-crunching programs tend to ignore Hyper-Threading.
So, Mathematica's probably just counting the 12 physical cores, ignoring the 12 additional virtual cores.