I'd say at least five-seven. Nomura only putting seven worlds and just adding one or two more worlds after the Mark of Mastery is really just a bad-case scenario I can't see happening. Also, is DDD releases late in March, who's to say they still can't put out one two more Famitsus in both February and March. You also have to account for the fact that there would be only one or two original KH worlds like Traverse Town, which I really can't see being the only ones in a series as major as this. There's only seven worlds for the Mark of Mastery, I really don't see what's the fuss with assuming there won't be more.So, how many are there left?
City of Bells
Country of Musketeers
Symphony of Sorcery
And I forget things because I not a genius, or smart.