Mid-2012 MBP -- put a 16GB SODIMM in one slot?
From Intel's website: 7th Gen (U/Y) and 8th Gen (U Quad-Core) Intel® Processor Families Datasheet for the i7 mobile processor.

Unlike the desktop counterpart that has support for 8 channels it (mobile chip) only has 2 channels with a maximum of 2 DIMMs per channel

Further in the datasheet, Intel specifically notes the maximumm amount of memory that can be addressed:



16GB is your max.