I think inititally it will be in the 50-100 range as the limit for restaurants/stores/etc. Even if the number was below that I think things any kind of team sports will be in a later phase. Kids soccer games probably before something like the nfl but after restaurants and the like.
Basically there needs to be a phased approach to it. Implement phase 1, if in a few weeks things look good then move to phase 2 and so on. Ideally the whole list of phases are released at the same time.
"Mass gatherings" are banned generically based on numbers because it is simple to do that, rather than to evaluate each on a case by case basis, which is what WHO recommends.
What looks like a "mass gathering," from the quantities involved, if managed properly (6 feet, temp checks, face masks) is not all that different than my grocery store, which after the governor of Ohio required limits on entry based on among other things square feet, now has a maximum capacity of 1,000.
MK could easily operate with 15,000 people in the park (after a lot of operational changes to make the 6 feet do-able.)
Another way to think of this is that the key is physical distance and ventilation.* Everything beyond those (mass gatherings, stay-at-home, work-at-home, close inessential businesses) is a way to make violations of physical distancing concepts less likely.
If you think of it this way, "mass gatherings" becomes a bit of a red herring in the arguments about whether or not the parks will open, and when. The issue is physical distancing.**
*You still need to act as though you are infectious of course--take your temp, stay home if you are sick. And you still need to protect yourself-- try not to touch anything you don't have to, wash your hands, don't touch your face between handwashings....
**My grocery store does not require temp checks or face masks, although the work setting I lead does, and you will get a funny look at the grocery store if you don't have a face mask on. WDW will be doing temp checks and face masks, I'd guess, based on the liability it otherwise creates for its cast members if they don't. Those not willing to abide by whatever rules are in place at the time, including whatever refund policies they promulgate, should stay home.