Low-level (1-5) are absolutely all over the place, the poor, the rich, the bored, the hungry and those who just don't mesh well with civilized society. Most of the adventurers here are young. "The Party" is likely to run into these guys in any given dungeon (either as corpses, prisoners or competitors).
Mid-level (6-10) are moderately rare. These are people who've figured out how to make a living out of what they do (think Indiana Jones), or are addicted to the rush and haven't stumbled into some bad juju yet. The wealthy and the bored are almost entirely gone from here. Even many of the poor have made it "big" and settled down. What's left are the glory hounds, the adrenaline junkies and the anti-socials.
Upper mid-level (11-15)are very rare. These are either heroic figures who have survives harrowing events or the anti-socials who are two steps away from being the target of the next adventuring party. The adrenaline junkies are usually dead now and the glory hounds have either changed their ways or retired.
High-level (16-20+) are exceedingly rare. You can count how many of them there are in the world on your fingers and have a hand left over. These people are the heroes that heroes look up to. Legendary figures who have done something incredible. (They are often also the villains).
For me this approach doesn't map well to the setting implied by the rules, and weakens our ability to deliver interesting challenges and constraints for PCs at all levels. Could I suggest that it makes naive assumptions that hinder rather than help our campaigns? In a way, our OP perhaps inadvertently built a misleading assumption into their question. I'll try to explain that below.
Tier 1 and 2 - Most core classes have implied organisations: barbarian tribes, bardic colleges, clergy, druidic circles, knightly orders, monasteries, ranger conclaves, thieves' guilds, arcane colleges. So we know that
first tier PCs are numerous. The OP typified these as adventurers and the thread ran from there, but many of these character class individuals are fulfilling a non-adventurer function. That's the misleading assumption: are we discussing only those who formally adventure, or all individuals with character classes? I believe the latter. So we have all of these organisations built around the existence of character class individuals to populate them. First and second tier PCs should not yet be at the head of those organisations.
Tier 1 and 2 characters are very common.
Tier 3 - That reserves such roles for third tier characters. We know that there are numerous such organisations - multiple arcane colleges for instance. So
there must be hundreds or even thousands of tier 3 characters occupying such roles depending on the size of your known world. Thus
Tier 3 characters are quite common - every city would contain several. Even small towns would contain one.
Tier 4 - Above such organisations are polities, which must wield sufficient force to cohere them. Rulers of a city or geographical area for example. So we know that
there are must be tens to perhaps hundreds of tier four characters. Again depending on the size of your known world. They would be
uncommon, but not rare.
Epic Tier - Where we come to a few characters - no more than a dozen perhaps - is at epic tier. Figures like the "Blackstaff" who runs (or ran) the very largest polities in our world (whichever world that is). I believe
epic tier individuals would be rare, no more than tens in the known world and perhaps fewer.
Thus I believe a "count up the murder hobos" approach is deeply mistaken. Instead we find a well rounded world that can engage our PCs for many levels by envisioning numerous character class individuals at all levels. You might say - but what of the simplified expert individuals such as spies, arch-mages and so forth? Mechanically those are almost all underwhelming: a CR 1/8th noble? a CR 2 priest? a CR 12 arch-mage? I feel that these are fine filling out mundane roles, but don't extend far enough up the hierarchy to create the maximum interest for PCs at all levels. As the game currently stands it is correct to envision numerous character class individuals in all the key roles. And that becomes even more true in the higher tiers.