Legions of angels

Matthew 26:53 - Do you think I cannot call on my Father, and he will at once put at my disposal more than twelve legions of angels?

I wonder how many angels there are. To the disciples, a legion would have conjured up thoughts of several thousand, perhaps 5000. So 12 legions would have been a number almost too large for them to imagine in one group -- 60,000. Certainly many more than needed to handle the current situation.

But are there billions of angels like there are billions of people? Is there (at least) one to watch over every individual -- Christian or not -- on the planet? (Nowhere does the Bible say so. I'm just wildly speculating on numbers.) Assume for a minute that there is. Does an angel get "reassigned" when someone dies? Or are there enough for every human that will have ever lived? Infinity plus 1? ;-)

I wonder how many fell from grace and now follow Satan. But I don't wonder much about that.