Space Marine Legions

Number of Legionaries
The Legions were massive armies, and the size of each could vary tremendously. A precise number was never truly achieved and maintained. Even during the Great Crusade some legions were very numerous, while others were not. The numbers would always vary with new recruits and inevitable battle-losses, and also important was the availability of potential recruits and the administrative skills of the Primarch and his officers.

The most numerous legion of all was the Ultramarines. The Thousand Sons of Magnus the Red were of a small number as many of them had developed mutations or uncontrollable levels of psychic powers. Fulgrim's samples had been largely lost, and this left the legion of the Emperor's Children also with a very small number. Both of these legions would increase their numbers to acceptable standards only after their Primarchs were found.

Original names
We must not forget that the legions were for a very long time under the direct command of the Emperor and that originally all recruits were from Terra. The Terran marines would always remember these times and the legions had Terran traditions. Later, as the Primarchs were found, the new recruits would have very strong loyalties towards them, and some of the primarchs would re-name their legions. The Emperor saw no problems with this development; the loyalty of the Primarchs to Him was believed to be unshakable. This reasoning would be proven to be sadly mistaken.

The first Primarch to be found, Horus, was the Emperor's favorite and he and his legion, the Luna Wolves were even granted the honor of being re-named as Sons of Horus. The First Legion was given the new name Dark Angels by Lion El'Johnson, and the Dusk Raiders received the name Death Guard by Mortarion, and while its original name is unknown, the highly honorific name Emperor's Children was awarded by the Emperor only after a speech made by their Primarch Fulgrim. It is unknown if other legions were also re-named.