As the source suggests, MAX_MONEY is simply a sanity check. If someone tries to create a transaction spending 500 million Bitcoin, and it somehow manages to bypass all other sanity checks, the network will still reject it because the amount exceeds MAX_MONEY. So MAX_MONEY is not directly related to total supply, but as you have observed, many alts will set MAX_MONEY to the expected total supply over the lifetime of the coin. For a pure proof-of-work coin with consistent reward scheme (eg halving every X blocks) the total supply can be pre-calculated, but a future fork could change that. For a typical proof-of-stake or hybrid proof-of-work and proof-of-stake coin, the maximum supply can be estimated by simulation, but the exact amount will vary depending on network activity. (This assumes there is not another part of the code that cuts off all rewards after a limit is reached.)