What does PCIe-x16 really mean and how many do I need?

2 is a slot that can interface with SATA , PCI Express

That mboard only supports PCIe 1, gtx960 is pcie but will work in the slot, however this means a slower data transfer rate = you wont be

pcie slot width Tech Specs General Product Type Riser card Expansion Connectivity Expansion Slots 1 1 x PCI Express x16 - full-height ; 1

pcie expansion slot SATA 3 Expansion Cards; Network Adaptors; Video Capture Cards PCIe x8 & PCIe x16 Slots: