My world has few "true" adventurers. There are many sell-swords and common mercenaries, pirates, and such. There are many people with skills represented by adventuring class levels. How they're represented, and how high are the levels they hold, is dependent on the locale; rural places have much lower-level NPCs than large cities, as a general world-design rule. But it's important for me to remember that, for example, while the village priest has the powers of a 7th-level Cleric he's never been in a dungeon in his life and would shudder to think of it.
The militia commander/guard captain might be a mid-level Fighter, though it's just as likely he's no fighter at all and instead has political connections. His sergeant, however, is definitely a mid-level Fighter, and has earned his Fighter levels the hard way. Again, he's never been in a dungeon in his life, and wouldn't for love nor money. He's earned all his XP in battle. Which is why he's a 50-year-old human who's still 5th to 7th level even though he's "...been in uniform one way or t'other since I'uz nowt but a lad."
Priests vary by location, as do arcane magic-users. Village priests are highly unlikely to be able to cast spells at will, though they can access rituals which have the effect of beneficial spells of healing and restoration and the like. A high priest in a major city is likely to be the equivalent of an Archbishop in the real-world medieval Catholic church, a person of immense political and temporal power but no adventuring-class levels. Adventuring priests are incredibly rare, as those devout enough to truly manifest their god's power as miracles tend to prefer lives of prayer and pious contemplation, and those with the drive to smite foes adventure as Paladins. But they do exist. They are usually "deployed" to adventuring groups for specific tasks. Same with Monks.
Wizards are highly organized. Education is largely accomplished by wizards. Even the most rural village has a hedge wizard who is the local schoolmaster. Her task is to identify those children with magical ability and ship them off to bigger, more challenging schools in the larger cities. Non-magical children are taught the three Rs and sent back to their parents. Non-magical scholars are also identified and sent on to higher schooling, usually resulting in bureaucratic jobs in government or guild. Adventuring wizards are also rare indeed, given the society in which they develop, and are also usually "deployed" to adventuring groups for specific tasks. There are a few who choose to endure the rough life, though they are usually held in suspicion by more settled, "normal" Wizards.
Sorcerers and Wild Mages are exclusively adventurers, even NPC mentors of PCs; in my world they're arcane magic-users without the benefit of formal schooling. The Mage's Guild discourages them, preferring to bring them in and teach them "proper" magic use. Those who refuse are held in contempt by the Guild. Though not as common as schooled Wizards, they're around. Warlocks are forbidden, on pain of immediate arrest, trial, and execution (there's history there involving a war and massive loss of life), so they're also exclusively adventurers. Life is interesting for Warlocks. Nobody's actually seen one themselves, but their brother's wife's cousin saw one a few weeks ago in an ale-house, that sort of thing. That's how rare they are.
All Bards are adventurers, though not all musicians/entertainers are Bards, if you get my meaning. Bards command a following at higher levels; they're always "on tour." There will never be two Bards in the same room without a fight breaking out.
In the same way, not all thieves and assassins have Rogue adventuring levels. Some do, but most do not. In fact, those with Rogue levels do best to hide them. Those with the Assassin archetype will never reveal it, as they are held in suspicion if not outright contempt even by their fellow criminals.
I can't put a demographic percentage on PHB-class "adventurers" in my world, but I can say, "precious few." There are no organizations or guilds of adventurers, though each group tends to name itself. The names tend to be as juvenile as garage bands.
PCs also have nemeses, in exactly the same way that Indiana Jones had René Belloq and Lara Croft had Alex West. It makes life more interesting for the PCs.