Does Notion Thief apply when an opponent's replacement effect would cause them to draw additional cards?

Notion Thief's ability is a replacement effect (CR 614.1) that modifies draw events: whenever an opponent would draw a card, they skip that draw and you draw a card instead. This applies to any draw an opponent would perform, including bonus draws generated by replacement effects — but only if those draws actually happen.

The key issue is replacement effect interaction (CR 616.1). If two replacement effects both apply to the same draw event, the affected player (or the controller of the affected permanent) chooses which replacement effect to apply first (CR 616.1a–616.1d). Once one replacement effect fully replaces the event, the other replacement effect has nothing left to apply to.

Concretely: if an opponent controls an effect that says 'if you would draw a card, draw two cards instead,' and Notion Thief is also on the battlefield, both replacement effects compete. The opponent chooses which to apply first. If Notion Thief's effect is applied first, you draw instead of them — the 'draw two' effect then has no draw event left to replace. If their 'draw two' effect applies first, the single draw becomes two draws, and then Notion Thief can intercept each of those two draw events, causing you to draw twice.

Example: Your opponent controls Alhammarret's Archive (double draws) and you control Notion Thief. On their opponent's draw step, they would draw one card. The opponent can choose Archive's effect first, turning it into two draws, then Notion Thief replaces each of those — you draw two cards instead. Or they could let Notion Thief apply first, and you draw one, with nothing left for Archive to double.

So Notion Thief absolutely can apply to additional draws created by replacement effects, but the order is decided by the affected player, which means they can minimize Notion Thief's impact by choosing the optimal application order (CR 616.1).