"Roughly 10 worlds" is not an exact translation of the original Japanese, the actual phrase used refers to any number between 10-19. The 10 number was arrived at by taking a statement from D23 about 9 worlds being divided into the 3 categories, plus a 10th world which is something that Nomura has always wanted to do in KH but has been unable to do before KH3. The 10 number seems to be for Disney worlds only, as there are 5 rumored/confirmed non-Disney Worlds in the game including Twilight Town, Mysterious Tower, Radiant Garden, Keyblade Graveyard, and Castle Oblivion/Land of Departure.