From the thread above, there is a link to @biduleOhm’s amazing work actually measuring drives, unfortunately the graphs seem to have disappeared.
Drives use both rails at spin up, but 3.3V is mostly irrelevant.
It’s very drive dependent. And up to 2A per rail per drive.
It’s a spin up current. It’s drive dependent. But doing staggered spin ups would help to.
FWIW, I have 24 drives + half a dozen SSDs and a Xeon 22 core processor, HBA, Expander, etc running on a Corsair RMx 1000W PSU just fine.