Quantum Riddler’s replacement effect applies to an instruction to draw more than one card before any replacement effects apply to individual cards drawn. For example, if you control Thought Reflection (an enchantment with “If you would draw a card, draw two cards instead.”) and you cast Decode Transmissions while you have one or fewer other cards in your hand, you’ll apply Quantum Riddler’s replacement effect first, changing the “draw two cards” instruction to “draw three cards.” Then Thought Reflection’s replacement effect will apply to each of those three individual card draws, causing you to draw a total of six cards.