Does Necropotence exile cards drawn during your draw step or only extra draws?

Necropotence has a replacement effect that modifies what happens whenever you draw a card: instead of the card going to your hand, it is exiled face down. This applies to every draw you make — your normal draw during the draw step, draws from spells like Opt, draws from abilities, all of them. There is no distinction between your 'normal' draw and extra draws under Necropotence's text.

Replacement effects (CR 614.1) modify how events happen. Necropotence's draw-replacement says 'If you would draw a card, exile that card face down instead.' Because it says 'a card' without any qualifier, it applies universally to each draw event you generate (CR 614.5 — the affected player applies the replacement).

The only way you get cards into your hand under Necropotence is by paying 1 life via its activated ability, which skips the draw entirely and puts a card directly from exile into your hand at the beginning of your next end step.

Concrete example: It is your draw step with Necropotence on the battlefield. You draw your one card for the turn — instead of it going to your hand, it is immediately exiled face down. Later you cast Brainstorm to draw three cards; all three are exiled face down instead of entering your hand. You would then need to activate Necropotence three times (paying 3 life) to eventually retrieve them.

This is why Necropotence is notorious for locking you out of traditional card draw — the replacement effect (CR 614.1a) intercepts every draw trigger or draw event without exception.