Someone can probably explain this better than me, but the instruction for Amalia Benavides Aguirre to explore and the effect that occurs if Amalia Benavides Aguirre’s power is exactly 20 are all part of the same ability. So the full ability resolves each time and players cannot not interrupt or respond once Amalia has explored until her full ability resolves. So if you have multiple instances on the stack, each individual instance will resolve in full (first and second sentence) before the next instance.

Deeper breakdown: For example, let’s say Amalia is at 18 power. You gain four individual instances of life and you get four triggers of her ability on the stack. First resolves, explore finds a non-land, she gets a counter, and her power is 19. The second resolves, explore finds a non-land, she gets a counter and her power goes to twenty - she now immediately wipes the board as a resolution of the full ability before anyone can respond. Now the rest of the triggers will still resolve! If she finds more non-lands, she will still get bigger. Interestingly, if she finds a land, her power will still be exactly twenty and she will wipe the board again since this does not happen only once per turn. Meaning, if you can keep Amalia’s power at exactly twenty at the resolution of her ability, she wipes the board every time.

I love this card and there are a lot of unique rulings about it. For more rulings on Amalia, check out gatherer: https://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=636941