There are, indeed, hospital systems that big, and bigger besides.
There is certainly some bracketing going on. When we're doing dogfights for true Enterprise accounts, we're doing them against publicly traded companies whose annual reports filed with the SEC says that > 60% of their annual revenue comes from less than 100 firms. Do the math on what that means their average account size must be for those 100 firms. It's nuts.
There are things you can say to me which will, quickly, get me to say "Look, I'll be honest: we'd love to have your business, but we're not the right fit for your organization. Can I give you the phone number of our #1 competitor? They're widely regarded as pretty trustworthy in the industry, and they're focused on helping customers who, broadly speaking, look like your organization." (Here's an example: "Does it integrate with SAP? Because we would need that." Ack, that will never happen, nope nope nope.)
That said, how can e.g. St. Jude Hospital (not a client, but representative of a few clients we have) use AR at the same time that Bugs Be Gone Exterminators (a 2 man firm) uses AR? Well, both of them also use Excel, right? Very different use cases, but same core need, and the sales model for Excel lets Microsoft offer it to both companies for an amount which is, for the exterminators, a trivial amount of money and for an amount which is, for St. Jude, so far below the noise floor it barely registers.
AR sells the same way, except it is a much narrower product than Excel is. To the extent that "It sure would be great if someone got a call from me" is the same problem for those two firms, we're equally capable of servicing it. (They have different needs, particularly with regards to privacy and compliance, which is why the hospital pays "rather more money than any of my published pricing suggests", but at the end of the day it is the same for loops executing the same state machine talking to the same Twilio APIs from the same hardware overseen by the same geek.)