The Apple page says "up to 12 cores" & intel XEON E5 chipset but the intel website shows only E5 processors up to 8 cores.
Like the Mac Pro itself the processor is a bit of sneak peak.
"... Chipzilla is said to also have a 12-core processor in the pipeline as well. ... "
http://www.engadget.com/2012/10/17/intel-roadmap-reveals-10-core-xeon-e5-2600-v2-cpu/
in the presentation I think they explicitly said "next generation Intel processor" , but they are not trying to be specific because they aren't suppose to be talking about it.
Apple's website is a bit premature not so accurate characterizing it as
" .. starting with the new-generation Intel Xeon E5 ... "
At this point technically it is 'the unreleased generation Intel Xeon E5 ..." Very similar to how the Thunderbolt 2 is also unreleased technology not due till substantively later in the year. It wouldn't be surprising if the GPUs are also unreleased speed bump versions.
It would not be surprising if the 12 core Xeon E5 v2 versions are not part of the initial release of E5 v2.