I have no idea. 10,000 seemed like a reasonable guess, though it's entirely possible they cut back on entries even more than that.
@sandam1 throws out some good ideas for how to mitigate the covid restrictions and impacts. Unfortunately, I doubt they're all workable for a race at Disney without a major overhaul of the race course and race logistics. The staggered starting times could certainly help cut down on crowding on the buses. Developing a course which allows for separate start/finish lines with plenty of parking at each while simultaneously allowing buses to get to/from each location from resorts is much more difficult. I'm not saying they couldn't do it, but my guess is they'd find it easier to offer up a Mandalorian/LBY medal and suck in money for a virtual event instead.