How can say in words 12 billion?

Assuming you're asking how to write that in a longer writing (essay, story, or term paper), it depends on where it is in the sentence. If those are the first words of the sentence, you need to spell out twelve. If it's anywhere else in the sentence, you wrote it correctly. Any word above ten can be written with the figures. Any word under 11 has to be spelled out. And the only time those rules change is the beginning of the sentence, which is why most writers don't start a sentence with a long number - it's too formal.

Then again, I just explained the rules for America. Every country has their own rules for this one.