If I choose not to use a Legend or Chosen Champion for Prerift Event, do I draw an extra 1 or 2 on my first turn?

If you do not use both a Champion Legend and a Chosen Champion in your deck, you draw one additional card at the start of the game.

According to RiftJudge FAQ #2684, the rule for extra card draws at the start of the game works as follows:

• You have both a Champion Legend and a Chosen Champion: No extra card is drawn.
• You are missing one or both (Champion only, Legend only, or neither): You draw one additional card.

Having neither a Champion Legend nor a Chosen Champion does not grant you two extra cards; you receive only the single extra draw provided by this rule.