How many Kromagg soldiers would need to be left on each Earth to keep it subjugated? The kromaggs aren't genocidal, they keep some humans alive as slaves. Our Earth has about seven billion people living on it. Even if half were killed it would require tens of millions of soldiers, maybe over 100 million, to keep the humans from revolting and overthrowing them. We have 65 million military and paramilitary personnel on our Earth now without it being a brutal dictatorship.
The kromaggs all come from one Earth, which they shared with humans. Assuming there are four billion kromaggs and half are soldiers (an absurdly high estimate, but whatever) they might be able to control a couple dozen dimensions if they could conquer them all. But each one they conquer makes it that much harder to take the next, as a large number of soldiers have to be left on the previous world. If they can conquer each Earth with a 2:1 troop advantage, since the worlds are caught unaware and the kromaggs have superior technology, they would still be able to take no more than about 20. They could expand that number if they concentrate on worlds with lesser technology and/or populations, but we've seen no evidence that they do that.
There's no way they could have conquered 150 worlds, as they claim, unless the vast majority of them are uninhabited like Outpost 113 originally was.