I have been building and messing with computers for years, but never with the option of this many drives.  It has usually been 2 maybe 3 at most, and that was years ago.  Mostly all of my systems have a ssd and a large data drive for storage (plus external backup).  That's it.  

 

I have the Norco 24 drive case, and powering up all the drives. I think there is 6 molex connectors.  Is it okay to run one cable that has multiple molex plugs? (fully modular PSU), or should a run a couple different cables and split up the power?

Use all the cabling you have available to spread the power evenly, rather than short-changing some of your drives when the load gets too high for a single cable.. Split each cable only as many times as you need to get all your connections made (if you need splitters at all).

 

In other words, if your PSU had 3 molex cables and you needed six connections, you should use two connections from each of your three molex cables. Spreading the power load decreases the voltage drop across the wiring (giving the drives more to use) and lowers power losses in the wiring (which therefore lowers the temperature of the wiring, though that is not generally a concern).