I've been looking at the numbers too thanks to this website:
https://teamrundis.com/rundisney-runner-totals
This is from the 2026 WDW Marathon Weekend, and the numbers are very similar for the past 3 years.
It does give a sense of what a person's odds may be when trying for a challenge or a specific race, for example, I was surprised by how few Goofy Challenge bibs there are.
This is the 2026 Princess Weekend:
My theory is that there are several factors that determine the numbers per race. One is time - how long the course can be open and that is probably constrained by park operations and road closures, Disney would want both of those to be as short as possible. Another factor is course capacity - how many runners can be on the course at the same time - a longer course means more runners on the course at the same time. If you take those two factors together as a formula (someone smarter than me might be able to figure that out) it probably would solve to the number of runners per race. I wish the 5k numbers were there too, my guess is that it's fewer than the 10k. And we would know how the new multiple start group format would affect that - basically they will do multiple 5k races with staggered starts, increasing the capacity.
The other factor, which a previous post alluded to, is price elasticity of demand, which for runDisney races (as well as a lot of other Disney World things) is very inelastic - meaning that the demand is so strong, they could keep raising the prices and they will still sell out. High prices may keep some customers away, but there plenty more to take their place. In this way, reducing the numbers post-2020 while raising prices would result in same or more revenue - fewer runners may equate to lower operating costs (i.e. fewer port-a-potties) and shorter course open times. Also, Club runDisney is a way to generate more revenue from selling the same thing, but offering an advantage for a fee (think Lightning Lanes).
The numbers look similar for Princess and Marathon weekend, but are lower for Wine and Dine and Springtime Surprise, which I the only reason I can think of is based on the slower sell out times meaning historical demand for them was lower?
Wow, I can't believe I typed all that out. I may be incorrect about any or all of that, but I do think a lot about the business side of Disney World and I find it fascinating.