It's 32GB, as limited by the CPU. But since there are two DIMM slots and RAM does not yet come in sizes of 16GB/module, you'd be maxing out at 2x8GB, for a total of 16GB.
Im just excited because I have a spare 2x8gb ram's here and this is my first mac and didn't know if 16gb is okay to put. Also on the apple's site they're mentioning only 8gb as max ram for my laptop's configuration.