How big is a Thieves' guild?

[Jd Smith1]

Generally, a thief needs a population of 1000 to support them. And single medieval realms tended to prefer corporal or capital punishment in lieu of incarceration, they would also need the population in which to lay low.

 

[aramis erak]

In a small city of about 10,000 people- how big should the thieves' guild be?

A rough estimate of organized crime in the US is about 850k in youth gangs. That includes the 3 in 5 who are adults.

Assuming another 150k in other organized crimes is probable... putting the ratio about 1 per 350 or so. BUT...
that's with the current 1.3% farmers. 90% of pre-19th century were farmers... a number stable at least as far back as the Domesday census of England.

So, since most of those gangs are urban, and 5/6 of the US are urban... ... that ups the ratio to about 1 per 300 urban dwellers.
This should vary by culture - the more authoritarian, the more organized crime. Peak rates run to aboput 1 in 50; above that, the parasitic effect is too strong and everyone starves. Minimum rates should be about 1/10, but figures on that are harder to figure.

So, from 1 per 3000 in an absolute democracy with a culture of honesty and fair enforcement of law, to about 1 in 100 for autocratic despotic cultures with no cultural traditions of honesty....

Of course, there's also the D&D solution in 3.X DMG...

1d8+6 for each of two "trees" - with 2x as many at half the level in each.
So a tree with a roll of 1 on the d8 is 1×7th, 2×4th, 4×2nd, 8×1st
A tree with an 8 on it is 1×14th, 2×7th, 4×4th, 8×2nd, 16×1st

 

[aramis erak]

Generally, a thief needs a population of 1000 to support them. And single medieval realms tended to prefer corporal or capital punishment in lieu of incarceration, they would also need the population in which to lay low.

Thieves' Guilds do more than theft... they are the majority of the organized crime.

 

[Jd Smith1]

Thieves' Guilds do more than theft... they are the majority of the organized crime.

Good point, but in a medieval society that doesn't amount to much. Gambling and prostitution is legal, there's usually no counterpart to narcotics, and so forth. Being a cash-poor society there's little opportunity for loan sharks. Basically the only OC options are smuggling t avoid taxes and custom duties.

And given such a period's absence of civil rights, crushing a TG that started getting too big would be simple.

 

[aramis erak]

Good point, but in a medieval society that doesn't amount to much. Gambling and prostitution is legal, there's usually no counterpart to narcotics, and so forth. Being a cash-poor society there's little opportunity for loan sharks. Basically the only OC options are smuggling t avoid taxes and custom duties.

And given such a period's absence of civil rights, crushing a TG that started getting too big would be simple.

Don't forget that the standard of living is much lower. And that members of the guild may also be performing legitimate business. The guild may also be the bank. Pawnbrokers would most likely be guildmembers.

And that, when Italy cracked down on the Sicilian Mafia, the mafia membership swelled, not dropped... and became a US intelligence asset and support recipient from the OSS.
The Egyptian crackdowns on the Islamic Brotherhood resulted in more such groups, and the Arab Spring revolutions.
The various groups are still growing, even as the revolutions have been put down...

Keep in mind that the big limit on thieves as a modern lifestyle is that they're selling stolen things to buy food.
Medieval thieves are often just plain stealing food to eat... which makes the issue not how many people are in the town, but how many people the farmers feeding the town can feed...
Protection rackets paid in food and shared to the guild; reduces two transactions.