Magical Subjects
Characters pick their best stat to determine how many of a given type of Magical Subjects. This gives the base number of hit-dice the character can control at maximum. This is then multiplied by a multiplier DEPENDENT on their level (but not actually their level). This refers to the TOTAL number, not any one spell - if a spell allows fewer, use the spell -for that casting- of the spell.
Thus a character is limited to a number of undead from Animate Undead but could, ultimately, have an army of the undead if they are a sufficiently high level with a good intelligence. It also means a character might have tons of low level characters doing their bidding - or a single major character.
Summoned Creatures (Intelligence, Charisma) Bound Undead (Intelligence, Wisdom) Controlled Constructs (Intelligence, Wisdom) Enchanted Thralls (Wisdom, Charisma)