Well prior to sh 1.2.0, the max sh can usually load is approx 70-80 dependent on the server.
With 1.2.0 and above you can load as much as your server can handle. (though it's not recommened running 100+ unless you like lag and other problems, not only that I won't support problems from running 100+).
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However, both are limited (to keep file/array size down) with a define to a max of 100 heroes. In order to load more than 100 you would need to edit the define in the include file and recompile the core plugin.
Code:
// Max Amount of heros that can be loaded
#define SH_MAXHEROS 100
for sh 1.2.0+ it is at the top of superheroconst.inc
for sh 1.18e or before it is at the top of superheromod.inc