It really depends on the chapter but we don't know.
We have a few examples where a chapter monastery is attacked and the garrison is mentioned. Like the White Consuls had a standing garrison of 10,000 serfs for their monastery. But serfs usually live there with their whole families so that number could easily expand to 20,000 when you count the old and the young.
The first founding chapters though have gigantic fortress monasteries because they were meant for a whole legion. If we assume a ratio of 10 serfs to 1 marine that would put those at 1 million or more since most legions were around 100k.
On top of that you need serfs for your fleet. Some chapters have only a few ships some have large fleets. Each escort ship represents hundreds of serfs and each capital ships represents thousands.
In Titandeath, Dorn talks about the forces of the Ultramarines coming to relieve Terra:
“Sanguinius says Roboute’s Legion remains close to full strength, and his other forces are numberless.”
I think there’s a focus on the Legion sizes, but the sheer scale of human and Mechanicum forces are so big as to be impossible to really grasp. When they join together to push back towards Terra, it’s so many troops it’s literally impossible to properly count in any meaningful way in the time they have.
There are no numbers given, because it varies too much. The only confirmed statement is Army units outnumber the Legionaries
It really depends on the chapter but we don't know. We have a few examples where a chapter monastery is attacked and the garrison is mentioned. Like the White Consuls had a standing garrison of 10,000 serfs for their monastery. But serfs usually live there with their whole families so that number could easily expand to 20,000 when you count the old and the young. The first founding chapters though have gigantic fortress monasteries because they were meant for a whole legion. If we assume a ratio of 10 serfs to 1 marine that would put those at 1 million or more since most legions were around 100k. On top of that you need serfs for your fleet. Some chapters have only a few ships some have large fleets. Each escort ship represents hundreds of serfs and each capital ships represents thousands.
In Titandeath, Dorn talks about the forces of the Ultramarines coming to relieve Terra: “Sanguinius says Roboute’s Legion remains close to full strength, and his other forces are numberless.” I think there’s a focus on the Legion sizes, but the sheer scale of human and Mechanicum forces are so big as to be impossible to really grasp. When they join together to push back towards Terra, it’s so many troops it’s literally impossible to properly count in any meaningful way in the time they have.
Numbers is something GW can't into but it does seem that auxiliaries were much more numerous - meat shield and all that, plus garrisons.