You seem to be conflating phones and phone numbers. I guess I don't know for certain that is not common to be juggling half a dozen physical phones, but it seems unhelpful and easily avoided.
They'll typically have an IT guy that handles things for the family. They'd be the one to set up call-forwarding and/or another tidy way of handling multiple numbers via one phone. Haven't done it myself but I'm sure there are numerous ways.
Even a free Google Voice account can manage some of that, but something like DialPad, (Made by the exGoogle engineers that made RingCentral which was bought by Google to make Google Voice) is a paid service that offers more features. If you think of being a celebrity as a business, who's product is that celebrity's name, time, and attention, then it seems obvious they'll have an IT department, a reception desk, a finance department, a social media manager, an assistant to manage their calendar (what's a calendly), etc.
Half a dozen phones's obviously a bit much, but if a phone gets dropped and breaks, the IT person should already have a replacement ready to go, no waiting around being phoneless while someone goes to go to the Apple store.