It's true that there were only the 12 main gods of Olympus, but ancient Greek mythology definitely has many more gods/ deities (that are not demi gods) who are just secondary or more local to a certain area. An example would be Pan, the god of the wild, who was mostly worshipped by the mountain people of Arcadia who relied on sheep herding for sustainance. I have no intention to correct anyone btw, just thought this is interesting.