1:100,000 is an unbelievably low number
Actually, even though "1 vampire per 100,000 humans" is so popular, there is a wide belief in the community (at least in Moscow, Russia) that this number is not that believable, or at least doesn't really make for a good game. Why?
Vampire: The Masquerade, as opposed to some other games, offers combat as your last resort in resolving conflicts (at least it is played here in Russia that way). Even though, also opposed to some other games, rules synergize with fluff text pretty well, combat rules are not what forms VtM, because this game is about talking in Elysium, not looting Dungeons and killing Dragons. Yes, even if you play Sabbat, VtM is still a game of politics and social interaction.
Of course, politics and social interaction require some kind of society to interact with. If you have your coterie of 4-5 players and your Story takes place in a small town of 1,000,000 people, you already have only 5-6 more Kindred to interact with, and that is really not much, especially considered that not all of them take part in politics. That would limit your play to large cities.
1:100,000 per what?
There is a claim, though, that the limit is not calculated per city, but rather per large area, or per country, or even per planet. So, for example, if there live 146.8 million people in Russia as of now, it means 1468 Kindred spread across Russia. And the spread does not have to be even: 1000 of them can easily live in Moscow, as finances are generally concentrated there.
One (hypothetical) group, playing at Rostov-on-Don (1 125 299 people living there, 11 Kindred by 1:100,000 rule) could imply that just all of the people from Rostov Oblast (population of 4,277,976 humans, 42 Kindred) form the limit from Rostov. This is slightly below the amount that normally plays there LARP (non-hypothetically), but many characters are assumed to be guests in Rostov.
Going further, China doesn't have "normal vampires", but 1,382,710,000 people live there according to Wikipedia. This could mean extra 13,827 Kindred spread somewhere else.
The number is not that important
In Russian language there is a set expression literally meaning "average temperature [of the patients] in a hospital". It means "useless average": if Kindred are spread so unevenly, it is important how many major characters you can meet somewhere (spoiler: exactly as many as you need for a good story), not how many can you (not) meet in a small village somewhere far from the place where your story takes place. And if your story takes place in a small village, if this place important enough, you will have some important guests: see the plot of Vampire: The Masquerade -- Bloodlines.
Vampires during Dark Ages
I cannot give you the source right now, but as I remember it, there were many more vampires before. They were stronger than vampires of the Final Nights, but their strength was more or less fixed, while humans did evolve further and further, gathering more knowledge about Kindred and crafting new weapons, those cool vampires were often too old to realize that process. And they died. Finally died.
Of course, not all of them were that stupid, some of them did understand how important the Masquerade is, and did invest huge effort in it.
But again, many died. If you are playing a Medieval campaign before the related events, you can just assume that there are a lot of vampires everywhere. For comparison, Los Angeles by Night has 23 major characters that are vampires. The action takes place in 1994, LA had population of 3,485,398 in 1990 and 3,694,820 in 2000, so around 34-37 Kindred should live there at all. At the same time, Constantinople by Night has 45 major characters, while I really doubt that there lived 4,500,000+ people in Constantinople in 1197. Wikipedia says that "Alice-Mary Talbot cites an estimated population for Constantinople of 400,000 inhabitants" about that time (would mean only 4 Kindred under "modern rules").
Who would count the population?
This question is very important. Nowadays major countries have population census every couple of years. It did happen before for the purposes of taxing, but our knowledge of High Middle Ages city population is actually very approximate.
And who would then calculate how many Kindred are there? How? Even if the almighty Camarilla can count its people, how would one count the amount of Caitiff, Sabbat, Anarchs (real Anarchs)?
Who and how will calculate the amount of Werewolves?
Conclusion: you do not need this number
Given all that, I want to say again that it is not really that important how many supernaturals are there altogether, because this number is meaningless and there would unlikely exist any real way to find it out in-game, especially in Dark Ages.
World of Darkness is not D&D. You don't build cities and roll dice to decide what is there for sale in a said city, you make a story with major and minor characters, with villains and heroes being perhaps designated, but more likely indistinguishable, and everything is adjusted for that story to be good.
You just don't really need those numbers for a good story.
Appendix: WoD LARP in Russia
I have heared that World of Darkness is LARPed very differently in Russia and in USA, and I want to talk a bit about how does it go with us.
First, the WoD community isn't too big, games don't happen too often, and even if they do, you rarely see new faces there. You don't either see a lot of new GMs, since to be a good GM in WoD you need to be a good LARP GM, know the world very well and be able to quickly fix lazy russians showing up drunk or not showing up at all -- I am not exaggerating here, this really killed some good games I have seen.
As a result, at any game you have as many characters as you have players enlisted, or some players don't find themselves any LARP event to participate at. Most people come in teams that form coteries or packs, and each team typically has an official leader. Many characters are typically foreigners. If a game takes place in Moscow, most players live in Moscow, but games in Rostov typically have a lot of visitors from all parts of Russia (mainly from Moscow, of course), because Rostov WoD community has some really good game masters.
Another important thing is that at LARP combat is an a lot more viable option, because most characters are more or less equal in power, and the most powerful combat chars have almost identical builds: the one who is better at fencing and attacking unexpectedly wins. The team I play with is damn good at both things, so our enemies rarely live long, and I have never seen conflicts really settled down without combat or our "conversation partners" realizing that if they don't say "yes", they die the next second (though, I guess, that is OK for a Sabbat pack plot-wise).
Foam weapons (like Calimacil ones) are used, but we hit really hard with them, and that is also an issue -- you don't want to go home with 6 bruises because a character can survive up to 6 hits by the rules and we didn't know that you can survive only one and have no muscles to protect your body under that cool suit.
So, not only is this number even more irrelevant at LARP, it is also impossible or at least very impractical to adhere to one.