Yeah...Roboute Guilliman decreed after the Horus Heresy that all the Legions (Which had upwards of 10,000 Marines at full strength) split into Chapters in the first founding. Each Chapter was to be 1,000 Marines in strength. This was so that no one man could control the might of a full Legion of Space Marines, and so the threat of rebellion on a scale such as the Horus Heresy was pretty much quashed (Although there was the Age of Apostasy and Vandire's Reign of Blood...but let's not dwell on that
)