6/27/2017 If you somehow have more than two main phases in a turn, each main phase after your first one is a postcombat main phase, and Neheb's last ability triggers at the beginning of each of them.
And here's the relevant section from the Comprehensive Rules:
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 yes, you'll keep getting mana after each of your extra combat phases (as long as Neheb is still on the battlefield, of course).
July 10, 2017 10:48 a.m.
Two things to keep in mind:
1) you can use mana to pay the first time. If just Neheb, the Eternal swings on an open board, your opponent will lose 4 life from combat damage on the first swing, so tapping 1 land to swing a second time will go infinite because on the second swing that will go to 8 damage.
2) combat damage is cumulative on creatures. If your opponent blocks Neheb, the Eternal with a 3/5 on the first combat then blocks with the same creature on the second combat, both creatures will die and your combo will end.