Beedo asked a good question about magical demographics in a comment to yesterday's post.
Before the internet I just pegged it at a 1000 to 1. So a city of 40,000 would have roughly 40 leveled Magic-users, and 40 Leveled Clerics.
As it turned about I wasn't far off the mark according to S John Ross Medieval demographics and other sources. There there are 1 clergy (people directly employed by the church) per 40 population and 1 Priest (i.e. D&D cleric) per 25 to 30 clergy. 40 * 25 means for every 1000 people there is a cleric.
My thinking on the ratio between Magic-users and cleric evolved from AD&D days to where I think for every 4 religious there will be 1 magic practitioner. This breaks down to
1 non-spellcasting clergy per 40 population
1 leveled cleric per 1000 population
1 non-spellcasting magic practitioner per 160 population.
1 leveled magic-users per 4000 population.
What is category three? Non-spellcasting magic practitioner? This this is the alchemist, store keep, monster hunter, component gatherer, etc, etc. They generally support the magic community.
For the Majestic Wilderlands I felt that the magic user ratio didn't produce enough magic-users. So I dropped it back to 1000 to 1 and put the cleric at 400 to 1 to keep them more common than magic-users.
Sean asked about the difference in play between AD&D style magic-users and GURPS Mages.
The main difference is that GURPS Mages are good at small scale spells. In terms of personal convenience and killing individuals; GURPS mage are far more powerful and versatile than AD&D magic-users.
AD&D magic-users are far more useful on the battlefield or dealing with large numbers of creatures. Many of the spells were developed during with D&D was a miniature wargame supplement and useful for in battle. Many spells developed afterwards continued to have large scale effects.
The patterns of rest and recovery are different for each. A AD&D magic-users has a lot more "stamina" over the short run but then comes a big down time of rememorizing from the spell books.
The GURPS Mage has short periods of intense activity followed by a short rest (less than an hour). In an all out fight a typical GURPS Mage will expend 10 to 15 fatigue some of it coming from powerstones (they store fatigue for spell casting). This results a half dozen spells cast in combat. Given the one second combat round and limited number you can cast due to fatigue it is important that you pick the spells for the situation. This compounded by the fact most of the spell work mainly on the small scale. Likely GURPS Mages can have a LOT of spells at their disposal making them very versatile.
In contrast the AD&D magic-users casting a spell is usually a battle-winner. A sleep or a correctly placed fireball can destroy the opposition.
For recovery have the Recover Energy spell which allows the restoration of 1 fatigue point per 5 minutes. Given the average mage has 10 to 12 fatigue this means they are full up in an hour. However Powerstones take much longer to recharge and are used sparely.