Are there more than 12 golden keys in Fairy Tail?
The Keys regenerate in time. This is confirmed early on into the final major arc of the manga, which has yet to start in the rebooted Fairy Tail Final Season Anime, but this is minor spoilers

https://fairytail.fandom.com/wiki/Aquarius in the Alvarez Empire Arc


  A year later, Aquarius reappears (of her own accord) from a spilled vase of water, saving Lucy from the wrath of (Spoiler)....Aquarius then proceeds to happily embrace Lucy, and explains that another Water Bearer key has surfaced somewhere in the world, and that the Celestial Spirit King opened her gate temporarily, which allowed her to save her


So its likely that someone did sacrifice the key to summon him, and then later on the key regenerated and reentered circulation.