In my world? That's a tall order...with no real answer.
In the megapolis free-city of Andril, you'll find adventurers of all levels, person's of note and power, multiple temples, thieves' guilds, wizarding centers, mercenary companies, entertainers, trained nobles, and retired types who've settled down into some nice "safe" business venture like merchant or innkeeper.
Other places you'd be hard pressed to find someone with any specialized training. Some areas you could go your entire life without ever seeing a spell caster...others you'd be hard pressed not to see impressive magic used every day.
Just depends where you are and where you go.
Typically, where the PCs are is going to automatically have a few...even rural areas. But that's more to do with the PC's being "unique and special individuals who live a life of untold wonders and adventure." i.e. What the PCs encounter in the day-to-day of their lives, is not what Joe Everyguy is going to run into.
As a general rule, for population centers, I try to use the "classic" 1-in-100 individuals have some class experience rule of thumb. Divide by 2 for each successive level, rounded down, and throwing leftovers/odds in on the top end. Then (the most fun part) figuring out who/what everyone is.
So, as some others have explained/exampled already, let's take a "typical small town" of, say, 7,500 people.
So we're working with a "classed" populace, not counting the PCs, of 75.
35 1st level NPCs:
17 2nd level (1 leftover):
8 3rd level (1 leftover):
4 4th level
2 5th level
1 6th level
Taking my two leftovers I end up with something looking like:
35 1st: I'd say most of these guys will be your town guard/deputies. At least half. Let's say 20 of them. 5 acolyte clerics that serve at the local shrine. 5 pickpockets getting their feet in the door of the local (very small) chapter of the thieves' guild. Let's say there's 2 neophyte druids in the near woods. 1 ranger-in-training among the local woodsmen. 1 illusionist trying to get their start in the good graces of a powerful mentor.
17 2nd: Let's say another half (8) of these guys are also part of the guard. 3 clerics at the temple (1 is a cult member). 3 thieves. The town's apothecary is a fledgling mage. And 1 former adventuring [fighter] innkeeper and (her spouse) the town's foremost entertainer [bard].
8 3rd: 2 constables. 1 eldritch knight, 1 cleric (making pilgrimage to the local shrine), 1 thief who are an adventuring trio that landed in town last winter and are still contemplating their next move. 2 priests in the shrine. 1 warlock (the affluent merchant's wife and cult co-founder).
4 4th: The town's sheriff (fighter). 1 head of the local thief guild chapter house (who also happens to be a representative on the board of the town's ruling council). 1 is the shrine's 2nd in command/cleric. 1 old hermit [monk] who lives outside the town walls (seems to have a good relationship with the druid and the ranger).
2 5th: 1 right hand man/bodyguard (maybe a barbarian?) of the town's mayor. 1 druid lives outside of town guiding the 2 1st level novitiates
2 6th: 1 ranger-turned-local hunter/trapper (most of the town is unaware of his advanced adventuring past, but he is training the young woodsman...his son perhaps? Or a nephew?). 1 "high priest/abbot/curate" of the local shrine to <insert campaign relevant deity here>
...and let's make 1 7th (cuz, why not?): an affluent merchant who is also actually a powerful Warlock setting up/leading an evil cult following for his fiendish lord.