Answer:
Along with the PDFs, Imperial Guard make up the vast majority of the Imperium of Man's armed forces. The Guard are divided into Regiments, which serve as their primary administrative unit from the point of view of high command. A Regiment will typically number between 2,000 and 10,000 men, although there are Regiments which defy this general trend, in both directions. Generally speaking, a Guard Regiment will have a specialised purpose, be they Infantry, Armour, Artillery, Stealth, Drop-Troops, Mechanised, or whatever. This has two purposes: it makes it easy for their commanding general to employ them to best effect, and, should any single Regiment turn traitor, it will likely be too specialised to be able to present a great threat.