If an effect instructs you to draw more than one card, Quantum Riddler’s replacement effect applies to that entire instruction, not the individual card draws represented by the instruction. For example, if you control Quantum Riddler and you cast Decode Transmissions while you have no other cards in your hand, Quantum Riddler’s replacement effect will apply, and you’ll draw three cards instead of two.

Once a replacement effect has been applied to an event, it can’t be applied again to the newly modified event. For example, once Quantum Riddler’s replacement effect has modified the “draw two cards” instruction of Decode Transmissions, Thought Reflection’s replacement effect can double the resulting card draws without Quantum Riddler’s replacement effect applying again.

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.