There's no single definitive number of "Greek heroes." The term "hero" itself is fluid in the context of Greek mythology. It encompasses a wide range of figures, from demigods like Heracles (Hercules) and Theseus to mortals who performed extraordinary feats of strength, courage, or cunning. Some were celebrated locally, while others achieved pan-Hellenic fame.
Therefore, any attempt to give a numerical answer would be arbitrary and depend on the criteria used for inclusion. Hundreds of figures could arguably be considered heroes in the broad sense.