About the Creed, it is about what we (or I) believe. Therefore, inside Creed, there is about the Holy Trinity. Besides that, Creed can be separated into twelve parts.
In accordance with an ancient tradition, already attested to by St. Ambrose, it is also customary to reckon the articles of the Creed as twelve, thus symbolizing the fullness of the apostolic faith by the number of the apostles.
(Reference: Paragraph 191, Catechism of the Catholic Church)
To know more about Creed, you can study Catechism.