OP is mistaken about the quantity.

It is not exactly 21 million bitcoins.  That is just an approximation.

The exact number of units that the protocol is designed to release is:

2,099,999,997,690,000 satoshi.

That's two quadrillion, ninety-nine trillion, nine hundred ninety-nine billion, nine hundred ninety-seven million, six hundred ninety thousand satoshi.

It will be easy for off-chain accounts to subdivide beyond that. It is entirely possible that, for off-chain accounts, you could transact in pico-bitcoins.  In that case you would have

20,999,999,976,900,000,000 pico-bitcoins (lets give them a slang name of picobits) to work with.

That's twenty quintillion, nine hundred ninety-nine quadrillion, nine hundred ninety-nine trillion, nine hundred ninety-six billion, nine hundred million picobits.