Welp, looks like I was right. Dusk form is one of a kind. Rotom had no data on it, and Kukui seemed to have named it Dusk form himself. So it's Ash-Greninja all over again with Ash getting another super rare pokemon. Remember when the most special pokemon that Ash go was a shiny Noctowl?
Nope, unless the Green Flash that occurs only occurs once every hundred years or something, then it's unlikely that Ash's Dusk Lycanroc is the only Dusk Lycanroc out there, as there are plenty of other Rockruffs that could have gone through the process as well, so chances are there are other Dusk Lycanroc it's just that Ash's is the first one were seeing and no Rotom having no data doesn't mean that Ash's Dusk Lycanroc is the only one in existence or the first of it's kind, as it's just rare and Rotom Dex doesn't know everything so it's just that it hasn't happened enough to have that many records but it's probably more common then the Bond Phenomenon.
And before you try to say that oh know Ash's is the only Dusk Lycanroc, may I remind you that if all takes for a Rockruff to evolve into a Dusk Lycanroc is the GreenFlash at Sunset or Sunrise occurring then it's unlikely no matter how you look at it that Ash's Rockruff is the only one that's gone through the process.