Asked in Yu-Gi-Oh
When should you put spell counters on a Yu-Gi-Oh card and how many counters can you put on per turn?
When you trigger an effect that says to put a spell counter on a
card that can accepts spell counters.
You can add as many counters per turn as possible, there is no limit. You put a counter on a card when the effect of a card tells you to do so (such as Breaker the Magical Warrior when it is Normal Summoned or Royal Magical Library when a Spell card is activated.)