OK I've done a bit of research, and this is what I've found.
Until recently, there were 6 Trappist breweries:
Chimay, Rochefort, Orval, Westmalle, Westvleteren (all in Belgium) and Schaapschooi (brewing "La Trappe" beers in Koningshoeven, Holland).
In around 2001 the only Belgian Trappist monastery that wasn't brewing, Achel, decided to sell off some land and resume brewing after 85 years to help pay the bills (the Germans had taken their kettles in the first world war).
This made for 7 Trappist breweries for a year or two until the abbot at Schaapschooi invited brewing giant Bavaria to take over the running of its business, to ensure its ongoing viability and so the monks could just get on with being monks.
They excused themselves from the International Trappist Association (the strict conditions of which they had actually been instrumental in setting a few years prior, to distinguish their product from that of the proliferating "abbey" breweries), as they were now technically a commercial concern. Apparently the monks are still firmly in charge of brewing (Bavaria has not even been given the recipe).
This means that we are back to 6 Trappist breweries, and they are now all in Belgium.
There are 2 other Trappist monasteries in the Association, Mariawald in Germany, and Tegelen in Holland, but neither seems to brew beer under the conditions of the association (namely that the beer must be brewed within a Trappist abbey under the supervision and responsibility of the monks, and the majority of the revenue must be dedicated to charitable work).