Homestead Arcana

there are a total of four spell slots (meaning you can unlock 3 more than your starting 1 spell slot) but to increase your number of spell slots you must consume a Tincture of Proficiency (each recipe comes from Abraham.. you need to buy the recipe for each one separately and one level of the recipe becomes available during each area you explore thus you'll be able to buy MK I while exploring the Pearl Canyon, MK II while exploring the Petrified Forest and MK III while exploring the Ancient Swamp) and creating each of these requires an Arcane Catalyst which are hidden throughout the game:

• 1 in Pearl Canyon
• 1 in Petrified Forest
• 1 in Ancient Swamp

if you need pictures of their locations https://homesteadarcana.fandom.com/wiki/Spells is a wonderful resource

the achievement pops once you use the final Tincture of Proficiency (MK III) to get your 4th spell slot