How many/much magic can one person learn in your world?

It is unlimited. Anyone can learn it, if they wanted to.

Magic is rare, but is still common enough that most every city has a mage-merchant/magic shop or a wizard for hire, and most rural regions have a few swamp witches nearby. Mages exist in all cultures. You maybe have a few dozen mages for every million people in any region, so somewhat commonplace, but rare enough that most individuals won’t encounter mages every day. A human may go their entire lives never meeting a mage in some regions or may see an entire group around town every week, depending on where they live and how heavily regulated it is in that region.

The Guild of Wizardry started centuries ago as a guild for mages, rules magic over the entire world, with the Justice Mages (Guild leaders) becoming political leaders (for example: governors, congress, etc in America; advisors to the Royal Family in England, that sort of thing - the series is set in an alternate reality of our real world, where magic has taken control of most every level of government and infrastructure, so while it’s our Earth with most of our Earth history - it’s also vastly altered and developed much differently, tech and medical are different for example - magic not electricity or gas runs homes and cars, cars use runes and gemstones to fly and you get the runes re powered at stations instead of putting gas in cars, and hospitals use potions and spells instead of vaccinations and pills and surgery is rare as magic can usually fix things without scalpels needed - that sort of thing; but, stuff like WW2, JFK assassination, and the Moon Landing still happened). It’s set in the 1970s because I published the first volume of the series in 1978 and set in the then current era, so it remains set in the 1970s even in the new volumes written today.

The world is a weird “steampunky-vibe”, but not steampunk setting that is a mix of 800 A.D. Quebec, 1640s Salem, 1800s Maine, and 1970s Old Orchard Beach, kind of all rolled into one, quasi-medieval hippie filled 1970s stuck in the past sort of weird alternate urban fantasy, that also has future tech from sci-fi like time travel, star ships, cyberpunk tech, and huge interplanetary portal systems.