How does Hour of Devastation's Neheb the Eternal work with extra combat & main phases?
Any Main Phase that follows a Combat phase is a "Postcombat Main Phase".


  505.1a Only the first main phase of the turn is a precombat main phase. All other main phases are postcombat main phases. This includes
  the second main phase of a turn in which the combat phase has been
  skipped. It is also true of a turn in which an effect has caused an
  additional combat phase and an additional main phase to be created.


So Nehab, The Eternal will trigger for each Main Phase that is not the First Main Phase.

It's also worth noting that the mana added is based on cumulative life lost by opponents during this turn. Any life lost prior to this triggered ability's resolution adds {R} to the mana added to your mana pool.