Yeah, your motherboard will be the determining factor in how many drives you can have.
My board has 4 NVME slots and a couple slots for 2.5 or 3.5 drives.
1 or 2 of the NVME slots disables a 3.5 or 2.5 slot i think.
Im currently running 4 NVME SSDs and 1 2.5 SSD drive
You can also get a PCIe hard drive expansion card that uses a free PCIe expansion slot! I recently picked one up for a single NVMe drive, but they also come in various configurations that allow for multiple drives! Prices will vary depending on size, but they’re relatively inexpensive!
Last time I remember, the Migration Software had a separate installer. But that may have changed.
I Run my Primary DCS from an external M.2 USB 3.2 Sandisk Ultra, that way I take it with me when I leave and run it from my laptop :-).
It reads/writes ~3000 mb/sec.
Anything SATA3 is limited to 500~ due to the interface limit.