Size of space marine chapter=1000 battle-brothers, approximately 1000 chapters are believed to exist at any one time
The first figure is bullshit, the second one a bit less so but also not very trustworthy. The number of marines is really not consistent, they die and get replaced all the time and chapters other than Ultramarines usually don't pay that much attention to Codex to go 1000 all the time. Black Templars run at few thousands all the time because simply they are so spread out and they give exactly zero fucks about Codex Astartes. The main argument for the 1,000,000 figure is that it all balances itself because some chapters also have much smaller number of marines due to Ork Snipers or whatever chucklefuckery they got themselves into, but honestly, there is proably much more than 1000 chapters, IIRC even the official estimates are at about 1350 chapters.
Also, honestly, it's not like GW writers give a fuck, if Matt Ward was still with us he would make Cato Sicarius pull few billion Ultrasmurfs out of ass and then solve all the Imperium problems forever, so Marines are more like constant nearly-infinite force.
As for the Great Crusade, iirc if you add up the values from various books you end up with like 2,500,000 Marines (That's the optimist number, but it still should be at least 2,000,000. Ultramarines had about 250,000, which is implied to include marines from the 2 missing Legions, Thousand Sons seem to be only ones with only 10,000 Marines... so yep, early days of Great Crusade is your only option to see such numbers of Marines working together). I don't know the exact numbers, but I'd say that at the end of Crusade about 500,000 would remain on Loyalist side. This number would be brought back to 1,000,000 during second and maybe few subsequent foundings. And then, there was 26 foundings and even considering constant losses and attrition, there should be way more than 1,000,000 Marines in Galaxy but very thinly spread out, with Eye of Terror encirclement being the place where you'd find most of them, but proably about 50,000 at best. Astartes Praeses had only 18 chapters, after all.
EDIT:
I'm gonna need some citation from a source on that.
Ultrasmurfs. Really just look here. The other, shittier wiki, also goes with these numbers and maybe has some more sources.