Actually it is not so simple as you initially can think.

Let's get for example AM4 consumer CPUs - Ryzen chips have 24 pcie v3.0 lines.
4x goes to NVMe slot of board (Intel lacks this). 16x go to GPU - and this one on A320 and B350/450 can't be split (aka bifurcation), to force MB makers to buy more expensive chipsets for SLI.
4x is left for the chipset (on Intel it is same, see below).

Now go into Intel consumer CPUs.

DMI 3.0, released in August 2015, allows the 8 GT/s transfer rate per lane, for a total of four lanes and 3.93 GB/s for the CPU–PCH link.

In August 2007, PCI-SIG announced that PCI Express 3.0 would carry a bit rate of 8 gigatransfers per second (GT/s)

So DMI 3.0 is PCIe 3.0 4x. And all this nice looking premium MB's actually share same actual 20x PCIe 3.0 lines.

Now back to our storage things with many NVMe drives. Ideally we want to have as much PCIe lines as possible to keep fast copy between any of them and allowing fast copy via 10Gb network adapter (and this one also eat lines a lot, you need PCIe v2.0 x4 at least for one port!).

For all chipsets that allow bifurcation of PCIe x16 GPU slot we must leave x8 only for GPU if it is editing/grading machine.
For storage machine it is best to avoid external GPU and go with build in one if you need it, so all x16 lines will be available.

For example Threadripper has 60 PCIe lines - x48 lanes for GPUs, x12 for I/O.

Now to our solutions, from top to bottom.

ASmedia 2824 based solutions. PCIe 3.0 x16/x8 into 4x PCIe 3.0 x4 or pair of PCIe 3.0 x4.

Expensive thing, $232

https://www.aliexpress.com/item/NEW-The-adapter-card-PCI-E-16X-TO-4P-NVMe-SSD-Support-RAIDO-PCI-E-16X/32951136605.html

Can also look at 2x variant - https://www.amazon.com/CREST-SI-PEX40129-Ports-Bifurcation-Controller/dp/B07HYZY7P2

Cheaper options requiring MB support

Asus Hyper M.2 x16 Card from $65 - https://www.amazon.com/Asus-Hyper-M-2-x16-Card/dp/B0753JTJTG/

ASROCK M. 2 SSD Upgrade Board from $120 - https://www.amazon.com/ASROCK-Upgrade-Board-Ultra-Quad/dp/B079TQ9C6Q/

These are dumb cards requiring MB to split PCIe slot aka PCIe bifurcation (and make proper RAID, if necessary).
On AMD it is usually for x399 Threadripper chipset only.

Cheapest option - use direct connection and one NVMe SSD per one slot

Can be with bracket or fully internal and compact. Best to use in x4 mode slots, so Intel chipsets usually work better (on AMD 3rd and more wide slots drop to PCIe 2.0 speeds).

Cots is below $5

• Fixable - https://www.aliexpress.com/item/H1111Z-Add-On-Cards-PCIE-to-M2-M-2-Adapter-PCI-Express-M-2-SSD-PCIE/32919000167.html
• Compact - https://ru.aliexpress.com/item/PCIE-M2-PCI-E-PCI-Express-3-0X4X8X16-NGFF-M-M-2/32948882740.html