An allusion to 21m bitcoins, the maximum number of bitcoins to ever exist.

ÄCKTSCHUALLY its going to be slightly less than that:

The Final Figure

Even if there were no trapped Bitcoins, it’s theoretically impossible to reach the figure of 21 million once all Bitcoins have been mined. In reality, the final figure will be very close to the Bitcoin supply cap. This is because the Bitcoin supply is never expressed in exact terms. Instead, the code Bitcoin uses rounds decimal points to the closest integer. As a result, a supply of 6.2589 Bitcoins is represented by 6 Bitcoins.

Bitcoins are split into smaller units, known as satoshis. One satoshi constitutes one 1/100 millionth of a Bitcoin. Due to these smaller units — and the rounding off of figures — experts suggest the Bitcoin supply cap will be limited to 20,999,999 instead of 21 million Bitcoins.