The main AP doesn't. There's a 'Continuing the Campaign' article that includes some ideas for further places to take the campaign for PCs that finished the main storyline.
For future reference, most official Paizo APs only go up to level 15/16 or so. I think because of a combination of word count limits and it becoming progressively harder to challenge PCs at that level and above.
None of the APs go past about 17th level, because once ninth-level spells and that degree of wealth (to name a few things) come into play, the party has the means to decide the direction of the aadventure beyond the ability of a book to easily restrict.