|Answer 1||We haven't been told. Only 17 keys have been revealed/mentioned in the series. (12 Zodiac Keys and 5 silver keys).|
|Answer 2||Sixteen Celestial Spirits have been introduced so far. There are twelve Zodiac keys. Lucy owns ten. And there are an unknown number of Silver Keys.|
|Answer 3||It should be noted that Celestial Spirits are named after constellations, which there are 88 of. Not only that, Uranometria also speaks of 88 stars; referring to the amount of stellar spirits, so there are 88 Celestial Spirits in total. 12 of which are gold keys - the zodiac signs (Aries, Aquarius, Taurus, Gemini, Cancer, Leo, Virgo, Libra Scorpio, Sagittarius, Capricorn and Pisces) and the silver keys being the remaining constellations outside the above.|
|Answer 4||In regard to answer 3: There are 88 types of Celestial Spirits, but of some spirit types there are multiple individual spirits. So far the only known case of this is the Canis Minor, since we saw dozens of those in the Celestial Spirit Realm and two different summoned ones (Lucy's Plue and Katja's Nikora).
In addition there is also the Snake constellation Ophichius, which isn't counted among the 88 constellations of Uranometria, but still has a key.