The last I heard is that Reed and Lucasfilm have an agreement for at least one more in the U.S. (hopefully in Orlando again, but no one will confirm). But they're not going to discuss it until after Celebration Europe II this Summer.
But if I were a betting man... I'd say Celebration VII will coincide with the release of EP7, and then they will continue the same with Celebration VIII/EP8 and so forth.
These actually started to be pretty profitable for Lucasfilm with Celebration V, whereas the first 4 were mainly marketing/promo ventures. There was some speculation that Disney would try to pull it into their D23 shows, but that's doubtful because LFL is operating independently for at least the next 4-5 years (plus there is the Reed contract). They will have to get Disney's blessing, though. But I don't see that being a problem. Also, the Disney connection may just solidify Orlando as the permanent home of the U.S. Celebrations (my personal speculation).
Celebration VII will happen. And likely more.