My opinion is that it's not the number of levels that is important, but the width and breadth of the complete narrative arc. A writer can state that expected leveling can occur at whatever spots they mention (especially in milestone leveling), so if it turns out they just only put forth 6 spots in the adventure where they say the PCs level up, it doesn't mean those six spots don't still encompass a full and complete story and path of adventure.
More importantly I'd say would be the number of discrete "modules" that are bound together story-wise that lead to a finale. One "module" wouldn't be a path, that's a point. Two "modules" could be called a path, but it's not much of one. Considering that a story is usually defined by having a beginning, middle and end... my own belief is you'd want/need at least three "modules" (or individual adventures) whose plots lead one into the next to be considered an "adventure path". But even then... that could be considered awfully short.
Were I to write one... I'd probably go with the seven-Chapter method. That gives you a much longer and wider story. You get a trilogy of adventures in Chapters 1-3 to establish the story and build rising tension, Chapter 4 is the midpoint turn of the story, then the final trilogy of Chapters 5-7 are building to the climax and the denouement. So seven "modules" all told at the very least.