It will use what ever lane (x16/x8/x4) your motherboard supports for the PCIe slot. This is usually x16 when running a single GPU. If you have multiple GPU's they can run using several different lanes (x8/x8) or (x16/x4). If your motherboard supports PCIe 3.0 and then it will run at x16 with one GPU.
x16 is the full bandwidth of the lane. Most cards cant fill this lane.
x8 is half, and some cards will be bottlenecked by this lane.
x4 will bottleneck most if not all cards to some extent. Avoid this slot.
Use the highest bandwidth you can, x8 isnt the ideal situation.