Some HP servers (that are actually smaller than an HP Z820) can fit up to 16 (yes, sixteen) SFF drives.
HP Z820
How many can a Z820 fit? Four in standard LFF (Legacy Form Factor) bays (see "8" in the illustration above) and 4-6 more, two per optical bay using an adapter like this:
That's a total of ten (eight if there's a DVD drive), while the system has fourteen SAS/SATA ports. You can see where we're going with this.
Why SFF (Small Form Factor) drives? For one, they're the present and immediate future of storage. SSDs are mostly SFF or smaller. SFF drives benefit from less vibration and can be made to higher tolerances. More SFF drives can fit in the same space which means higher speeds when you stripe them, which in turn means you may not need a heavy, bulky, expensive external SAS expander box to house a bunch of drives for those uncompressed 4K workflows.
Enter HP 4-in-1 SFF (2.5in) HDD Carrier (P/N B8K60AA) retailing for $139 and what really is Icy Dock MB994SP-4S. Says so on the label.
This metal device lets you use four SAS or SATA spinning or SSD drives, and has two fans and four SAS ports in the back, in addition to a single Molex 4-pin power connector.Two of those, and you can have a total of 12 drives in the system. Not quite server grade: HP server drive trays are much, much nicer, support hot swapping with the right controller, and their activity light indicate possible fault and RAID set identification, besides, well, activity. This unit's activity lights are tiny, hard to see, and in my experience, don't always work. We've asked HP to come up with server-grade backplanes for the workstations and still hoping it might happen.
HP Z820
