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If you call "n" the nth square, there are grains in the nth's square.
So in the last square there is grains. To see how big or how small this number is, go there: http://www.wolframalpha.com/input/?i=2^63.
I don't really know the number of rice's grains in the world, though it shouldn't be so hard calculating it. If you can give us the rice world production in tons by year, we will have the order of magnitude of the number of rice grains in the world, assuming that 1 kilo contains a certain number of grains.
Notice also that on the 63 th square there is half of the grains in the 64th square and that you also have to sum it up.
This number (of rice on the last square), as huge as it may look like, doesn't scar me. Indeed, there is 100 000 times more atoms of lead in a mole of lead, namely a small fragment of lead that you could have in one hand.