With cryptocurrencies, there are often set amounts of coins that will ever be minted, this is known as the max supply.
A coin like Bitcoin is well known for its max supply. It is set at 21 million coins meaning that there will never be more than that many Bitcoins in existence. However, the max supply is not always that simple to determine for some coins that have an ongoing supply.
If we use Bitcoin as an example, its maximum supply is 21 million, but that number will only be reached in a number of years. Bitcoin's deflationary nature means the coins are slowly released over time until the maximum supply is reached.
Maximum supply is different from the total supply, and circulating supply. Again, with Bitcoin, the maximum supply is 21 million, but the total supply would be how many coins have been released up to this date - falling short of 21 million. Circulating supply would be total supply subtracting how many coins are lost or unreachable.
The reason that many cryptocurrencies have maximum supplies plays into how the market for that coin works. Having a solid and well-structured issuance rate together with a predefined maximum supply can be valuable for controlling the inflation rate of a cryptocurrency. Inflation control can potentially lead to a long-term appreciation of the coin.
It works on simple supply and demand as when the maximum supply is reached, there are no new coins entering the market, and there will be fewer coins available on the market. This is expected to create market scarcity, which may eventually lead to deflation conditions and increased demand and value.
For other chains, like Ethereum, there is no set maximum supply as coins can continue to be minted indefinitely.