Prickle Faeries’s ability will check as each opponent’s upkeep begins whether that player has two or fewer cards in hand. If they have three or more, the ability won’t trigger at all. If the ability does trigger, it will check the opponent’s hand again as it tries to resolve. If the opponent has three or more cards in hand at that time, the ability won’t resolve, and Prickle Faeries won’t deal damage to them.