Complex sentence: As an exception, the first cohort consisted of the bravest men from the legion and had only 5 centurias, but since these were double centurias (160 men) the size of the whole cohort remained the same.
Split into simpler sentences: As an exception, the first cohort consisted of the bravest men from the legion and it consisted of only 5 centurias. However, these were double centurias (160 men) so the size of the whole cohort remained the same.