It depends on the motherboard and what slots you populated.
If there are multiple PCIe x16 slots, at least two of them will be connected to the CPU. In this case, the number of lanes gets distributed between them to the closest power of 2 up to the most they need. So if you have an x16 card in one slot and a x1 card in another, the x16 card will only get 8 lanes, even though there are 15 available after the x1 card uses what it needs.
For the rest, they typically disable storage ports. Some motherboards with multiple M.2 slots will only let you use the M.2 slot or a specific PCIe slot. Others will disable a pair of SATA ports.