This is not that complicated. If you set your server to 15 slots, then get_maxplayers() returns 15.
As mentioned, MAX_PLAYERS is set to 32 which is the max number of players that a HL1 server supports. This constant can be used for sizing arrays since you cannot use a non-constant, such as get_maxplayers(), for this purpose. The value returned by get_maxplayers() can be used for run-time conditions, such as max iterations for a loop or something.