PCIe is designed so that in theory basically any device can connect via any number of lanes with the bandwidth limited by whatever is lowest.

You can put an x1 Card in an x16 slot and it will work fine and you can put an x16 card into a slot that only has 8, 4 or 1 lanes and it will work albeit with only half, a quarter or a sixteenth of the bandwidth. In theory you could file open the end of the slow to allow cards with longer connectors in shorter slots but a riser seems like a smarter idea in practice.

Many motherboards have a second x16 slot that actually only works with 8 lanes and only is x16 in length so that people can put larger cards in there without having to file anything down.

Generally the number of lanes in total even with risers is limited by the PCIe controller which in modern computers is in the CPU and in slightly older ones in the chipset. Even server level Intel Xeon CPUs only provide 40 lanes worth of PCIe per CPU.