I always thought that the maximum number of lanes for a PCI Express link was 16. The most powerful and expensive gaming GPUs never offer more lanes, and I have never seen a motherboard featuring a slot longer than 16 lanes. But is it really 32 lanes?
Support for 32 lanes
Reading the lengthy article on Wikipedia there are numerous quotes supporting up to 32 lanes:
The PCI Express link between two devices can vary in size from one to 32 lanes.
The PCI Express standard defines link widths of x1, x4, x8, x12, x16, and x32.
Consequently, a 32-lane PCIe connector (x32) can support an aggregate throughput of up to 16 GB/s.
A connection between any two PCIe devices is known as a link, and is built up from a collection of one or more lanes. All devices must minimally support single-lane (x1) link. Devices may optionally support wider links composed of 2, 4, 8, 12, 16, or 32 lanes.
Support for 16 lanes
But the same article also says that 16 is the maximum in many places:
Physical PCI Express links may contain from 1 to 16 lanes, more precisely 1, 4, 8 or 16 lanes.
Width in bits 1 per lane; 1–16 bonded lanes
A PCI Express card fits into a slot of its physical size or larger (with x16 as the largest used)
In real life
Meanwhile, a typical high performance graphics card is probably the most common mainstream expansion card to need a large amount of bandwidth. Enthusiasts pay several hundreds of dollars for these cards and often buy two or three. High-end consumer motherboards feature several PCIe x16 slots for this purpose. As we can see, there is obviously room for a longer connector on these cards:
A full-size ATX motherboard can also comfortably fit a connector that's twice as long.
There is also a continuously increasing demand on per-lane bandwidth. It tends to double for each new PCIe generation.
Question
With such a high demand of increased bandwidth, manufacturers competing to deliver motherboards with more and more PCIe x16 slots, millions of gamers paying hundreds of dollars each to get the absolute fastest graphics cards out there, why haven't I ever seen a PCIe x32 slot? Is there something in the standard preventing this?