Does Eager Apprentice's effect stack?

Yes, Eager Apprentice's effect stacks if you have multiple on the battlefield.

According to the official FAQ: "Ruling: Yes, Eager Apprentice's effect stacks if you have multiple on the battlefield."

Important Nuances:

• Stacking: Multiple Eager Apprentices stack their discounts (e.g., two Apprentices provide a total cost reduction of 2).
• Minimum Cost: While their discounts stack, energy costs cannot be reduced below zero (or the specific minimum stated on the card). If you have multiple Apprentices, any total cost that would result in a negative number is treated as zero.
• Defy Interaction: Defy reads the printed cost of a spell, not the modified cost. Therefore, cost reductions from Eager Apprentice do not change what Defy considers for its effect.