

Really depends on what you want out of the system, what you can spend and how much time you want to spend on it.
My old z390 itx system has a 16x PCIE to 4x m.2 card - leveraging an m.2 to 5x SATA adaptor with the built in SATA adaptors has given it plenty of space.
Considering I can grab m.2 to 6 SATA adaptors and fill the remainder of the slots that’s a decent chunk of drives from a single PCIE x16 slot.
Software is another kettle of fish and a good way to timesink, I’d rather not give too much of my personal experience as there are so many ways to skin that cat.
I believe that functionality exists already if you are using Plex; via RSS sync.