Yeah, if your audience was shared from Analytics, being over 20 million uniques would definitely cause a problem. I don't think there is a limit on real-time audiences (created in Audiences Library or AAM) because they are passed to the edge as a set of rules rather than a batch of IDs.
As for the max number of Audiences, each client can have 20 segments shared from Analytics at any given time per report suite. I'm not aware any variations of that number. Just to be sure, I'll double check with product management and get back to you.
Update: Analytics Premium customers do not get any additional shared segments. Each customer can have 20 Analytics segments per report suite shared with the Experience Cloud. Real-time Audiences created within Audience Library and AAM do not have a limit.
Maybe you're confusing it with Customer Attributes? Limits for customer attributes below: