On Windows there are two options:
1) Get an already made DLL that supports more.
2) Modify your DLL's source files (it is apparently just changing one number in one location) and rebuild the DLL. This is more complicated than it sounds since it involves installing software and it doesn't go smoothly for some people.
If you are on a Mac, you don't. It simply is not an option. (Although I'm not certain that there is a Mac version that has 24 civs, at least one of the mods shipped with it has more than 18 so you may be talking about that.)