A lot of modern boards come with 3 or more - that's what mine has. And with modern density, that's a LOT of storage. I have two 4TB drives!

You could even get more using a PCIe NVME expansion card, since it's all over PCIe anyways.

My desktop motherboard has 4... not sure how many you need, even if 8tb drives are pretty pricey. Though actual PCIe lanes in consumer CPUs are limited. If you bump up to ThreadRipper, you can use PCIe to M.2 adapters to add lots of drives.

On top of what the others have said, any faster interface you replace SATA with will have the same problem set because it's rooted in the total bandwidth to the CPU, not the form factor of the slot.

E.g. going to the suggested U.2 is still going to net you looking for the PCIe lanes to be available for it.

Three is not pitiful. Three is plenty for mainstream use cases, which is what mainstream motherboards are designed for.

The MSI motherboard I use has 3, and with the PCIe expansion card installed, I have 7 m.2's. There are some expansion cards with 8 m.2 slots. You can also get SATA to m.2 devices, or my fav is USB-c drives that hold 2 m.2. Getting great speeds from that little device.

Most consumer motherboards have 2-3 M.2 slots.

You can buy cheap add-in cards to use PCIe slots as M.2 slots, too.

If you need even more slots, there are add-in cards with PCIe switches which allow you to install 10+ M.2 drives into a single M.2 slot.