45 billion is the total eventual supply of ADA (up from from 32 billion or so in circulation now, of which over 71% is staked).
What elements of the code of the Cardano protocols and the structure of the ecosystem ensure that 45 billion is the maximum number of ADA that will ever be in circulation, and when will that maximum be available in circulation for all purposes such as staking etc?
You cannot guarantee that the supply is constantly 45 billion Ada for all eternity, however; you can say with certainty that this particular version of code doesn't support increasing the supply. At some point in the future, the majority of block producing nodes (SPOs) could fork the code base and increase the supply with a future hard fork, but that's the case with any decentralized blockchain.
