6x8TB here. Adding 2x more when I can (complicated). I'm old school though, so Ive got an
LSI card running Raid 6 with a BBU. And the whole server is protected by a sinewave UPS. Can withstand two drive failures at once (hopefully that never happens) since a Raid 5 is completely vulnerable when rebuilding after 1 failure - hence why I went with R6.
I'd say the biggest points to think of in general are: how many drives will I ever need (future proofing)? And what do I do in case of a disaster? For many, their media is already backed-up remotely or at least duplicated off-site, but it varies. The first part though, space... drives! I've got room for 12x 3.5" HDDs and 4x 2.5" HDDs/SSDs, but that required finding an older case with many, many 5.25" bays (10x 5.25 bays!). But I use 8TB drives which are quite affordable nowadays, if you go with larger drives, you can obviously get away with fewer of them.
All I know is, with all the ripping and other content I've added to my file server, I went from having about 10TB of free space pre-covid to less then 2TB of space now, so in 4ish years, I added more than 25% more content. Space runs out fast... and I've barely even begun to rip my 4K stuff

, definitely gotta fix my FS up and expand my Raid, and Soon!
Hope it helps.