Make it so that you never forget spells, but you have a hard limit on the number of spells you can know at any time (except possibly for Wizards; the limit could vary from role to role). Not specified if you could still learn new spells and forget old ones to make room like move sets in Pokemon, or if you’re locked into knowing the first N spells you learn in the game. It’s also OK for amnesia effects to cause spells to be forgotten.