They ARE officially limited to 1000 per chapter, by the Codex Astartes, but chapters lik the Wolves and Dark Angels (I think) are non-Codex compliant, and don't bother with keeping to the thousand marine limit.
Yep -- and on the flipside, some Chapters may have far fewer than 1,000 due to casualties. All in all, maybe it evens out, so that the one million still makes for a good estimate, give or take a few thousand.
The SIsters, well there isn't even a standard size for the Orders Militant, however, there are exactly 6 Orders Majoris, and we know they all number in the thousands. I think it would be safe to assume that none of the undisclosed number of Orders Minoris have more than any of the Orders Majoris.
It's about time you get to read that 2E codex! It actually goes into detail about their numbers nicely.
The six Major Orders Militant have a nominal strength of about 5,000 Sisters each, with the two Primary Convents (Prioris on Terra and Sanctorum on Ophelia VII) capable of accommodating 15,000 warriors each, but by the end of M41, their numbers have dropped to about 3,000 to 4,000 due to how difficult it is to replace higher casualties incurred in recent major conflicts.
The Minor Orders are said (in the 5E core rulebook) to normally have about 100 Sisters, but this number can vary tremendously, from a single squad guarding a relic/shrine or patrolling a pilgrim route, up to a Commandery of ~200 Sisters as mentioned in the 3E Codex Witch Hunters (some might even have Preceptory-size!). However, since the number of Minor Orders is not established, this does little to quantify the size of the Orders Militant as a whole.
The only hints we have about the overall size would be the fact that it took the Sisters of Battle 2,500 years to grow from a force of 10,000 to 30,000 (late M38, when Deacis VI decreed the foundation of the Orders of the Bloody Rose and Sacred Rose), so that we can extrapolate a "growth rate" that may possibly be used to calculate the rise of the Minor Orders up to M41 -- and GW's Force Disposition Charts about the 13th Black Crusade and the Third War on Armageddon, as originally published in Codex: Eye of Terror and the Armageddon 3 global campaign website and its accompanying issues of White Dwarf, but re-printed in the 6th Edition tabletop core rulebook.
As you can see, the number of Battle Sisters present compared to Space Marines is somewhat negligible.
My personal (very rough!) estimate would be that there is maybe about 100,000 Battle Sisters around, their comparatively small numbers in good part due to how difficult it may be to find a human being who is at least somewhat capable of keeping up with an Astartes, provided you give them the right equipment and lots of training and indoctrination.
Edited by Lynata