Also bear in mind Kryptons mastery of the life sciences. In the modern more dystopian Krypton they had eliminated all diseases and were virtually immortals, hence they felt little urge to procreate and expand. If you simply can combine DNA and create life in vitro their race can be ten in one generation and a billion in the next..

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Plus, why would Clark think it worth leaving Earth with its billions of people for such a small number of NKers?
Because they are his people? whom he apparently has hereditary obligations to?

I do know you, and I know you wouldn't lie... at least to me...most of the time...