I'm running Mathematica 10.3.1.0 on a machine with 16 cores, but Mathematica (in the Parallel settings) says "Number of processor cores is 8." It's a 16-core Opteron 6300 chip on an Ubuntu system. All 16 cores show up in the system information and function normally within other applications.
I manually set it to 16 (with no error or warning from Mathematica), but running parallel operations throws several errors about kernels seeming to be "dead."
(Note that the license is not a problem. The limitation displays correctly as 16 cores.)
Additional Data: Thanks for the preliminary responses. I am sorry I was not more specific. This is an Opteron 6376 chip, model OS6376WKTGGHKWOF.
To the best of my knowledge (which could be wrong!) this chip has 16 physical cores. I can't find more specific information on product pages, manuals, etc. that lists anything for the number of cores other than "16."