If you have "Heart of the Underdog" face-up active on the field, and you have "Exodia the Forbidden One" in your hand, any other cards in your deck is normal monsters including the four limbs of Exodia. During your Draw Phase, you can draw infinite amount of cards until you gather all 5 pieces of Exodia or you can just use its effect to draw once for the card you draw at each draw phase?

Provided all cards left in your deck were normal monster cards, you could draw each of them until you either A) have all parts of exodia, or B) unable to draw.

Also, your opponent can chain the to activation of HOTU activation if he/she wants to.