Depends on how the lanes are split, it could be 16x and 8x, but more likely 8x and 8x with other lanes for high speed connectivity (10Gbe, NVMe drives etc)
12x is not an option, only 1x, 2x, 4x, 8x, 16x. Fun fact PCI-Express can go to a max of 32x, but the connector would be huge and there is not a need for it.