According to Bruckheimer himself:
Pirates of the Caribbean 4
At a recent junket for his upcoming release Glory Road, Jerry Bruckheimer expressed his interest in continuing the Pirates franchise. If it were up to him he would continue doing more. As for the third film being titled “Unchartered Waters”, its pure rumour for now.
I guess it depends on how deep and creative the next two are, but I think three is typically a nice number to round out a really good set of movies. But who knows… with same cast and movie staff, maybe we could break the mold and set new standards with a whole series of high quality movies.