How much mana does tapping a Forest make, with Nikya of the Old Ways & Nyxbloom Ancient?
Nikya's ability doesn't cause the Forest to produce {G}{G}; it still produces {G} and Nikya's triggered ability produces another one. You don't tap Nikya for mana, so Nyxbloom Ancient only applies to the original {G} from the forest and you make 4 green mana.

Now, two Nyxbloom Ancients would stack, cf. the following ruling on Mana Reflection:


  The effects of multiple Mana Reflections are cumulative. For example, if you have three Mana Reflections on the battlefield, you’ll get eight times the original amount and type of mana.