How do I know I have to use more than one hss file?
You never need to use more than one HSS file. The only reason you might want to break up your script files is if you want to organize things better or, for example, you're are using a self-made battle engine which can make your hss file rather big. This depends on the situation and is a matter of taste.
Typically, very large sets of scripts can be much nicer to work with if they are modularised into different files. You can put all your global variable declarations into one file, for example, so that you can find them easily.