Theoretically, all the councillors could be co-opted!
Remember, under law, if a vacancy occurs, then that is notified to the District or County Council, they then put up the notices asking if people want an election to fill the seat.
If the electorate does no say they want an election for that vacant seat(s) then the seat(s) can be filled by people being co-opted on.
If that happens often enough you could end up with a PC made up mostly of Co-opted Councillors or even one made up completely of Co-opted Councillors. (I know of two councils where all the members are co-opted)
So it is not about what is 'reasonable' it is about what the law deems reasonable, and that comes down to the electorate. Nor should you fall into the trap assuming that co-opted councillors are in any way less able the elected councillors, often they are not.