There has only ever been one King of Wales. That was Gruffydd ap Llywelyn, from 1055 to 1063.
And that's it. Wales will never have another king. The current monarchy is for the UK, not for Wales. And the national party of Wales (Plaid Cymru) is not in favour of retaining the monarchy in general.
There is absolutely no possibility of Charles, or any of his family, becoming monarch of Wales without also being the monarch of the UK.