If your maximum addressable market is “the whole economy,” as seen in SpaceX filings, then a city-sized call centre (distributed, of course) really is ‘t that much of an ask.
If you think you hit a limit where you can't support any new customer anymore, you just tell them you have no capacity at the moment. Like any normal practice does.
What's the support staff required for a few billion people?
Their problem, not ours.
And yet, building city-sized datacenters is somehow totally doable.