I can only find low quality information Reddit about where the Legions membership were drawn from before the Primarchs were found, but it is worth thinking again about the scale of the Imperium in the 40K universe.
Terra itself is home to billions.
Terra is perhaps the most massive Hive World within the Imperium, with an uncounted population that numbers several hundred billion human beings. The bulk of Terra's massive population is divided between the upper class that includes the Imperial nobility and Adepts of the Adeptus Terra and the masses of the lower classes who serve as basic labourers.
The elite fighting force of the Space Marines represents a much smaller number of individuals.
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.
That page gives an estimate of around 2.2million.
It is possible that all space marines were going to be recruited from earth.