A Legion was made of ten cohorts, each one made out of six centuriae of eighty men. Like in Camp Jupiter, the most prestigious one was the first, then the second and so on. Normally the Centurion of the first Centuria commanded the whole Cohort.
Later, when Polybotes went to attack Camp Jupiter with his army, the Fifth and Second Cohort were trying to surround him. However, Polybotes caused the two cohorts to take a severe beating until Percy interfered and eventually defeated him.