With Charles as king, I can see him downsizing the working family to just his own family and his siblings. But it may also be feasible for his siblings to retire too. If the new King and Queen, Prince and Princess of Wales, and Harry and wife do 500 engagements each, then that would be a sufficient number to keep the public happy. Plus the duchy of Lancaster would only have to support 2 extra royals, rather than the current 8.
Harry has already indicated that he wants to stay in the army until 55 or so - or for another 25 years - so he won't be on the full-time roster for a quarter of a century.
He may also never marry. He does appear to have trouble finding a woman who wants to stay with him for very long, as other than Chelsy, his relationships barely last a year or so, so it is possible he may never marry. If he does why should his wife have to give up her life to support his family while he continues in the army anyway? Kate didn't have a career to give up as she hadn't ever started actually working but surely Harry's wife, if he finds one, will be a woman who has had to work for a decade or more and thus has a career of her own.
There is no way Camilla will do 500 in a year - she has never done it and as she is approaching 70 now will never be expected to do so.
It is normal for royals, from around 70, to begin to slow down a bit (The Queen was over 600 regularly in her 60s but around 400 ever since she turned 70 or so)
400 would be the maximum you could expect from the royals in their 70s -and that would be the monarch and spouse and heir and spouse only if over 70.
10 years from now Anne will also be in her mid-70s so would also be slowing down.
Andrew, Edward and Sophie won't be increasing their numbers from around 300 to 500 as they move further from the throne.
William and Kate will have to be at around 900 or so a year to maintain the current numbers of around 4000 as Harry will be limited to around 100 as a full-time officer in the army and as he becomes increasingly irrelevant (as Andrew is now) it would make more sense to let him stay in the army for as long as possible so that he doesn't end up like Andrew - despised and seen as a waste of space as the next generation become the focus of the nation's attention (it is what happened with Margaret and Andrew and the only reason it didn't happen to the two previous generations was that Edward VIII didn't marry and have children so his 2nd brother remained relevant - but his younger brothers - particularly the Duke of Gloucester was despised as the years went on) and the same with Victoria's younger sons.
I don't think we can take anything for granted, especially the relationship between siblings. A perfect example is Prince Laurent of Belgium. Here was someone visibly estranged from his parents, his siblings and the demands of royal life but, with the ascension of his brother Philippe, suddenly he is the "go to" guy. He has Philippe's back just as Philipe has his. He is in a position that no one could have imagined possible.
