When you look at the limitations it seems like it would be 300k users because you can have:
30k users per distro group
100 total distro groups per app
2000 apps per organization
But there's other weird limitations like:
1. Maximum number of members in an organization: 5,000 members
In this case are members in the organization different than members in the distro group? What's the max number of people you can distribute to using app center's plans? Does it differ on free/build/test? Doesn't seem like it, but want to make sure.