Nephalem Valor, or NV is sometimes also referred to as "stacks" or "NV Stacks" because you can stack the effect up to 5 times.
5 happens to be the magic number, regarding quite a few things in Diablo III. In order to have ANY possibility of a Keywarden to drop a key or Infernal Machine plans, or any of the Uberquest bosses to have a chance at dropping a body part for making a Hellfire Ring, you MUST have 5 NV stacks. The Monster Power level of the game also affects the drop chance, but without 5 NV, the chance is always ZERO.
With MP0, your chance to get keys or body parts is 5%. With MP1, your chance becomes 10%. With each increase in MP level after 1, you gain another 10% chance for keys or body parts, so long as you have 5 NV at the time of the drop. This means that at MP10, there is 100% chance to get the special drop from those bosses.