1) Hundreds of Millions.
2) Theoretically nothing, if you discount checkpoints. with checkpoints it's hard.

But then there's the social 51% attack where a tiny majority hold a massive percentage of the currency. When this occurs the market is open to extensive manipulation for the benefit of the few, as with real world economics (the 1%).

NXT is a good example of the social 51% attack, the top 33 accounts hold 51% between them. The top 50 accounts hold 61.2%. I'm quite sure the top 1% of accounts (400 ish) hold almost everything, with the other 99% playing with spare change. source

I think that's why you see so many NXT shills trying to ram it down everybodies throats on this forum.