What wire would I need for 24000 watts (48v @ 500amps max) of DC power?
You are deep into busbar territory at 400 A.
You should consider multiple wires unless you want to get into very large cables.

Amp capacity of wire is generally conservatively rated at 700 circular mils per amp as in this table.

You could use multiple automotive primary wire connections. This is good multistranded insulated wire that is flexible enough to route. You'd need 4-5 cables for 400 A continuous.

Or if you could use uninsulated cable, then automotive earth braid might be a good choice. If your cables are only a few inches long, this might be the ideal choice as a single run would support 380 A continuous, and well over 400 A burst. You could put loose insulation covering over it.