you can get as many necklaces as you want, you just gotta get lucky on the rewards RNG when rolling for it.

If you dont wanna slog through getting all of the stones, fragments and forms in order to get guaranteed necklaces that then have a 1/4 chance (or 1/6 if you havent alr gotten the light + dark necklaces) of getting the golden skull necklace, i'd recommend using the Mystic Assistant mod that allows you to manually purchase specific necklace types