It IS kind of a debatable point about the canonicity of the other players you see in the world. Tongue-in-cheek dialogues will say they're your adventurer friends and are lore-accurate, but as ARR jobs took pains to establish how rare these arts are and how forbidden they are and how no-one else would ever be using this fobidden magic anywhere... (likely to make the jobs feel that more special over their base classes) It ends up painting that immersion into a corner (ans, as has been previously debated endlessly, the race/gender combinations are not exempt from this, such as a newbie will see a flood of Au Ra around Coerthas Central before they're at the point that Ishgardians consider maybe not shooting them on sight. Hell, the PC may be one of those).

Outside of a handful of Jobs that have positioned the WoL as the unique practitioner, or at least unique-outside-of-specific-race (WHM and SCH come to mind), the rest find some lore justifications: You can headcanon White Mages as simply random conjurers who just so happen to know Holy... for reasons. Or Alphinaud's unique Academician job can serve as a headcanon for other Scholar players. Or maybe X'run ended up training other people and didn't mention it, or other Red Mages that eventually trained their own pupils that show up in Bozja. GNB actually deals with other practicioners who were away from Bozja and ended up training their own disciples (Thancred among them). While MCH is specifically recruiting new members at all times. Dancers don't seem to have a problem bringing in new people, either. Even Summoner brings in more people with the ability to Summon in their story, if you get enough job crystals (of which there is a surplus in the SMN job quests) you just need to go on an expedition, absorb some of the constantly resummoned Primals' aether, and you got yourself a new Summoner, with the added benefit that apparently everyone in Eorzea got a healthy dose of Bahamut-flavored aether, so even Dreadwyrm trance is rando-canon.

Bottom-line is: uhhh, try to ignore other players as much as possible? And for RP purposes, I guess people would just need to get on the same page? If you're decidedly going full-in-universe canon then not everyone can be the only Non-Padjali White Mage, or one could handwave it for RP purposes as a cabal of "Not-White-Mages" closer to Eastern Geomancers.

As for the actual question, I'd think the most likely to have a bunch of people running around with job stones would actually be Blue Mage, given how they're actually SOLD, alongside a spellbook and a cane. After that, possibly Machinist, Monk after the events of 4.0, and probably Dancer; with the special snowlakes being White Mage, likely Scholar, and certainly Dark Knight. The rest sit somewhere in the middle as "rare but not incredibly rare, just not common to Eorzea"