From the lexicanum:

The Emperor's Children developed a very rigid combat methodology that was reflected in their order of battle. By the time Fulgrim led them into rebellion his Legion comprised 11,000 marines organized into 30 Millennials, the first ten of which were led by a Lord Commander. As each Space Marine looked to his superior officer for guidance, each Company inherited its manner and practices from its Commander. Though this was the case with many Legions, the Emperor's Children had a strength of devotion to their leaders that was almost unmatched

There might be more precise information in Horus Heresy Book 1 from forgeworld, but I don't have it so I don't know.