What are the exact rules for defender the magical knight in the Yu-Gi-Oh TCG?

The card lore says "When this card is Normal Summoned, place 1 Spell Counter on it (max. 1). Once per turn, if a face-up Spellcaster-Type monster(s) on the field would be destroyed, you can remove 1 Spell Counter from a card you control for each of those monsters instead." Does that mean it can only deflect one attack (battle or spell/trap) per turn? Or can it deflect every spell-caster you have face up on the field once per turn? Or something completely different?