Note: The description of the question contains some crucial spoilers, so I decided to put the whole question in a spoiler tag just in case I don't spoil for some users that haven't watched the anime or read the manga.
This is a question that struck me after completing the anime. It has been revealed that the more blood he gives to a demon, the stronger they become, with the Upper moons holding the highest ranks. But it was never explained how does he decide to which person/demon he will transmit his blood to? Does he have some kind of ability to see through each person's potential? Also, why doesn't he give a big quantity of his blood to many demons other than the 12 moons so that way he will have a way bigger and stronger in quantity demon army and make sure he never gets defeated?