So if 800A OCPD could be satisfied with two sets of 600 kcmils (820 amp) = 1,200 kcmils, then having six sets of 500 kcmils = 3,000 kcmils which is 3,000 / 1,200 = 2.5 increase.
There is some ambiguity to using this approach--in 75C copper, 800A ampacity could be achieved with 2 sets of 600 kcmil, or 3 sets of 300 kcmil, or 4 sets of 3/0, or 5 sets of 2/0, or 6 sets of 1/0. Those are total kcmils of 1200, 900, 671.2, 665.5, or 633.6, respectively. Depending on which of those 5 options you take as your base case, 6 sets of 500 kcmil would give you a upsizing multiplier of anywhere from 2.5 to 4.7.
Your choice to compare to 2 sets gives you the smallest multiplier among those options and hence the smallest sized required EGC of 264 kcmil. If an inspector argued that since you have installed 6 sets, the proper base case is 6 sets of 1/0, so your upsizing multiplier is 4.7, so you need to use a 496 kcmil EGC in each set, it would be hard to rebut that based on the ambiguous wording of 250.122(B).
Cheers, Wayne
P.S. I guess you could also compare to 2 sets of 900 kcmil all in a single conduit, so now your multiplier is only 3000/1800 = 1.67, and you only need 176 kcmil EGC, or 3/0. If you are free to compare to any configuration of your choosing.
P.P.S. Under a regime where the AHJ requires you to compare based on the number of conduits you've actually run, and the least cross-sectional area option for the required ampacity, you could reduce the required EGC size by running fewer conduits (plus you'd need fewer parallel EGCs to boot). For example, for 2 conduits the options are 2 sets of 600 kcmil, or 4 sets of 250 kcmil (0.8 ampacity adjustment), or 6 sets of 3/0 (0.7 ampacity adjustment), for total areas of 1200 kcmil, 1000 kcmil, or 1007 kcmil. So now the worst case multiplier is only 3.0, and you'd only need a 317 kcmil EGC.
