Ice, Dragon, and Ghost are all around 30. Really no reason to exceed that straight out the gate. 25-30 is pretty likely.
+3 Marill line (if the baby form changes from normal) +2 Jigglypuff line +3 Gardevoir (under the unproven assumption of including Ralts and Kirlia, but not Gallade) +2 Clefairy line (safe assumption IMO) +1 Sylveon +1 Flabébé
That's already 12. If Flabébé has two more pokemon in its evo line, then that's 14, which could honestly be half of all fairy types and still be the size of ghost type. Honestly, I kind of have a gut feeling there will be more than that, but honestly there's no real reason to believe that at this point. It's really going to depend on how many pokemon get retyped. GF could be conservative with just 4-5 families, or they could retype every "fairy-like" thing ever, including the Chansey line, Misdreavus line, ect.
Umm, if you are considering the possibility of having Azurill with Fairy type, why not Cleffa and Igglybuff as well? It makes the same amount of sense.
Either way, people seem to be way too excited with the Fairy type, looking everywhere for candidates to get the type. I don't think that would be a realistic expectation.