Just one Universe?

Assuming that God exists, that 'he' is omnipotent and that he has created this universe with all the necessary parameters for us to exist in the way we do, why couldn't he have created several more universes before or along ours? Although maybe with totally different parameters resulting in very different forms of intelligence?

Or couldn't he do that and our universe is the only viable one?

Necessity of a Creator?

Christians claim that an external creator has created the Universe because somebody had to create it. But God on the other hand didn't, according to them, need to be created. 'He' just was or came into existence on his own.

But if it's possible for a God to exist without a prior cause why would it be impossible for a universe to exist without cause? Why couldn't the Universe just have created itself (via the Big Bang for example)?

And why couldn't God be the Universe and vice-versa (pantheism)?