How are there so many necrons?

So the necrons likely have numbers similar to the Imperium, which is to say, quadrillions. But how do they have so many?

From my understanding, the Necrontyr were a bunch of sickly and short-lived nerds stuck on one planet, and were mad jelly of the Old Ones for being long lived Chad's, so made a, deal to have their souls stuck into robo bodies.

So how are there so many of them? Even if their planet was comparable to an Imperial hive world, they'd only number in the billions. And since they can't reproduce or create more bodies, how did their numbers inflate so much?