If you want some of the most insane storage you can cram into a PC, then you will want to check out ASRock's new Hyper Quad M.2 PCIe 4.0 Expansion Card.

ASRock's new Hyper Quad M.2 PCIe 4.0 Expansion Card if you couldn't already tell from its name, can handle 4 x NVMe M.2 SSDs and cram them into a single PCIe 4.0 x16 slot. It supports M Key type 2242/2260/2280/22110 M.2 PCI Express modules.
The 4 x onboard M.2 sockets are on a 45-degree angle inside of the card, a move by ASRock to keep the traces as short as possible -- meaning that the M.2 sockets have enough current, something that is imperative for the speeds that PCIe 4.0 SSDs can reach in the multiple GB/sec range.

It has Bootable M.2 RAID support, where you can have up to 4 x PCIe 3.0 x4 NVMe M.2 SSDs on the AMD TR40/X399 platforms, as well as Virtual RAID on CPU (VROC) technology on Intel's competing X299 platform (Intel SSDs are the only ones that support VROC).
There is also a cooling fan to keep those ridiculously fast PCIe 4.0 SSDs nice and cool, with a 5cm fan keeping temps as low as possible. The fan in question is adjustable, with ASRock including a switch on teh PCB or you can use the included software to do it from Windows.

If you're like me, you're a speed freak -- so ASRock includes support for having multiple Hyper Quad M.2 PCIe 4.0 Expansion Cards in the single system. Imagine that!