This seems to be a case of Ron using hyperbole. We know from elsewhere in the texts that there are only 12 OWLS taught at Hogwarts12 OWLS taught at Hogwarts. While it's theoretically possible that there were more subjects in the past (or that there are vastly more OWLs than those that are normally taught), the reality is that he was almost certainly exaggerating for effect.
For the record, we also know that the giant squid is actually quite pleasant and doesn't really need anyone saving from it.
