The Maximum Supply of a cryptocurrency pertains to the maximum number of coins or tokens that are in circulation and those that have yet to be created. It is differentiated from “Total Supply” which only takes into account what had been produced and is in circulation, not considering the coins that have been destroyed through coin burn events.
What this means is that should the maximum supply be reached, new coins would no longer be mined, minted or produced in any way.
The Maximum Supply is limited by what is determined by the protocol that each digital asset takes cues from. As this is the case, the maximum supply and issuance of new coins are solidified through the Genesis Block in accordance to the source code of a project.