Well, there's one militia per 20M, and they're supposedly 10 large planets in size, so that's what, 40B people? That would translate to 2000 militia.
My experience suggests that even with decent large troops, it's best to drop more troops than militia if you want to take the planet in one turn. Keep in mind that the first (several) wave(s) might get completely annihilated by sheer numbers...
It might take you less time (6 years) to build your own sphereworld than to train and gather those troops.
So, basically, it'd be easier to use PPP, if you're not willing to bring out the planetary napalm.
Or, bLast it to Deadly using contamination ops, sabotage the facilities, and use anarchy Groups so it riots.
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-- The thing that goes bump in the night