The "Ask the Staff board" is supposed to be used for problems or requests about the wiki and community, not for the show itself. If that's what you wanted to ask, go to the Steven Universe board. That is the forum to be used for any questions about the show.
But, to answer your question, there are most likely more. Spinels may be rare, but I doubt there'd only be one in existence. As for why more haven't appeared yet, there are two likely reasons:
1. Because the Spinel we know is supposed to be special, unique, and be a Gem we've never seen before. She's meant to stand out, so having multiple Spinels just running around would cheapen the intended effect and leave Spinel just another Gem type.
2. It just didn't fit the story they were trying to tell. Keep in mind, the movie and Future (the only pieces of SU media Spinel has appeared in so far) are very tightly-written stories, with a clear goal and purpose in mind. Sure, the original series had that too, but it also had 5 seasons and 160 episodes, meaning it had time for, for lack of a better term, fluff. Compare that to only an hour and 22 minutes and 20 episodes, and you see the difference.
The film and epilogue series had a very clear and limited story to tell, so there wasn't really time for stuff like introducing more Gem types of an already-existing character, especially the movie, where the "Spinel being special" effect was in full effect. And Future was about Steven's slow mental, emotional, and physical deterioration and transformation into a monster (yeesh, that sounds dark), so it also didn't have time to introduce more Spinels.
And there you have it.
(I've moved the thread)
Woah, you can do that? Nice.