Well you need 13 for Ultima Weapon but there are only 7 in the game. So how can you achieve it then? You use a Serenity Crystal. Easy when you know how.

The seven can be obtained as follows:
1. Seal Atlantica.
2. Seal the 100 Acre Wood.
3. Clear the Goddess of Destiny Cup in the Underworld Olympus Coliseum.
4. Collect one of every synthesis material.
5. An item box in Twilight Town: Sunset Terrace
6. An item box in The World That Never Was: Brink of Despair
7. An item box on Space Paranoids: Central Computer Mesa.

Hope it helps