These days monarchies are curious things, IMO. There is a need for a head of state as a symbol of national unity on the world stage, and to perform those actual powers the duties the head of state in a constitutional monarchy has, such as opening parliament, assenting to legislation, appointing ministers and receiving ambassadors and foreign heads of state. These tasks have to be performed, whether after hobbling in a horsedrawn carriage dressed up in fine gold and silk clothing or just driving there without fanfare wearing ordinary day dress. This is where the decision has to be made as to not whether but how these tasks are going to be performed.
The British head of state is a monarch who has lots of fancy carriages and lots of money and property and other accumulated wealth of jewels and other treasure... most of which actually belongs to the people. The monarchs can only retain their elevated social position if the people allow it, and the people are going to get really cheesed off if they don't get to enjoy all their goodies by seeing the jewels and regalia on show. The opening of parliament has been described as "an elaborate ceremony showcasing British history, culture and contemporary politics to large crowds and television viewers" and is one of those events where the public's property is put on show for them to enjoy, so is unlikely to stop, along with all the other elaborate events with carriages and pagentry and bagpipes and fancy uniforms. The people enjoy seeing this stuff; it's great entertainment and it's good for tourism.
I do not include the ribbon-cutting and handing out of prizes at local fairs, and much if not all of the charitable work the various family members perform (apart from the heir, who is in a separate category, because he has to keep himself busy and useful and is separately funded) as part of the core business of the monarchy. I see it as really only busy work to keep the royal family members occupied and give the people a connection to the royals.
Charles and William are going to have to decide how much of that busy work is going to be retained: how many charities are going to have royal patrons, how many bridges, fairs, etc., are going to be opened by a member of the royal family and how many trees planted by them. When they have decided what work has to be done, they can work out how many working royals are going to be required in order to perform that work. Or maybe they will firstly decide the number of working royals they want and then assign the tasks. Either way, a decision is going to have to be made.