Well, this can't be true if there are a million worlds in the imperium but only 50k sisters of battle
In the least there has to be millions if not billions. At the very least every world with a billion or more guardsmen has to have a few hundred thousand SoB.
I think the developers can't do math basically. They've forgotten how big the 40k universe really is and how many planets the imperium of man occupy. I believe I've read somewhere that the Sisters defend Ecclisarchy churches and important places, the Ecclesiarchy are also amongst the first on new worlds occupied by humans. Even if there was just a single battle sister on each world with some Ecclesiarichal presence there should be hundreds of thousands of SoB.