If the National Museum of Wales (or possibly the Tate) hasn't got anything planned, you might have a long wait for an exhibition, since it's usually the galleries that hold a large number of works to start with that arrange exhibitions (by, where necessary, borrowing further works from other galleries or private collectors).
You might have to settle for viewing the 14 works at the Tate:
http://www.tate.org.uk/servlet/ArtistWorks?cgr oupid=999999961&artistid=1363&page=1
or, for a far greater number of works, visit the National Museum of Wales.
The NMW holds a very large collection of works by Agustus & Gwen John. If you make an appointment to view, you won't have to wait for an exhibition (i.e. you'll effectively get your own private exhibition). See here:
http://www.museumwales.ac.uk/en/108/
Chris