No amount larger than this (in satoshi) is valid.
Note that this constant is not the total money supply, which in Bitcoin currently happens to be less than 21,000,000 BTC for various reasons, but rather a sanity check. As this sanity check is used by consensus-critical validation code, the exact value of the MAX_MONEY constant is consensus critical; in unusual circumstances like a(nother) overflow bug that allowed for the creation of coins out of thin air modification could lead to a fork.