As I understand it Haswell CPU's offer 16xPCIe datalanes. These can be divided into a single 16x slot, two 8x slots or an 8 and two 4's. There are technologies that can get around this issue but they don't multiply up the datalanes since this is a function of the CPU.
As it stands only LGA2011 CPU's offer the most number of datalanes (40) and they are supported on X79 Chipsets.
Until recently that meant losing a lot , but modern boards like the Rampage IV Black Edition (this is what I use) have most of the typical modern features associated with Z87.