Hello prestotron,

18446744073709551616 is the number of atomic units ('satoshis') that is emitted with proof-of-work. As it is the maximum value that a 64 bit type can store, you can't make the total number of coins greater without changing PoW or without implementing PoS/PoA or another concept. But you can make it less, if you want.

How can I find out how many are left?

Here's the formula to calculate the number of coins in circulation (

An):

,

where

n is the current block height,

q is emission speed factor and

M = 2^64 -1

Number of coins left for mining is calculated as a difference between total number of coins and number of coins in circulation.