The short answer is there is no good answer.
That being said look here at discussion of reasons for multiple power domains.
Its a bit art, not science.If your ports lightly loaded, current and C, you may be able to get
away with sharing a coupling cap across several Vddios. If you are doing high precision
analog and driving lots of C and I, probably not.
Not all coupling caps are equal, look at their specs, especially z vs f. MLC and Polymer tanatalum