If multiple creatures with lifelink you control deal combat damage at the same time, the damage dealt by each of those creatures is a separate life-gaining event and Amalia Benavides Aguirre's ability will trigger that many times.

Amalia Benavides Aguirre's power must be exactly 20 during the resolution of its last ability after it finishes exploring in order to destroy all other creatures. If it's more or less than 20, nothing is destroyed.

Amalia Benavides Aguirre's power is checked only once, as the triggered ability resolves. If it is no longer on the battlefield at that point, use its power as it last existed on the battlefield.

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. Players do not get to chance to respond to the ability after knowing the result of the explore.

If an ability instructs a creature to explore, its controller reveals the top card of their library. If it's a land card, they'll put it into their hand. Otherwise, they'll put a +1/+1 counter on that creature, then choose to either leave that card on top of their library or put it into their graveyard.

Once an ability that causes a creature to explore begins to resolve, no player may take any other actions until it's done. Notably, opponents can't try to remove the exploring creature after you reveal a nonland card but before it receives a counter.

If no card is revealed, most likely because that player's library is empty, the exploring creature receives a +1/+1 counter.

If a resolving spell or ability instructs a specific creature to explore but that creature has left the battlefield, the creature still explores. If you reveal a nonland card this way, you won't put a +1/+1 counter on anything, but you may put the revealed card into your graveyard. Effects that trigger "whenever a creature explores" trigger as appropriate.

Some spells or abilities might cause a creature to explore multiple times in a row. If you reveal a nonland card when a creature explores and leave it on top of your library, then the creature explores again immediately afterwards, you'll reveal the same card again.

In some unusual cases, noncreature permanents may explore. For example, if the creature card returned by Defossilize is somehow not a creature once it's on the battlefield, it can still explore. You'll take all the same actions, and you may end up putting a +1/+1 counter on the permanent. (Note that some effects target a creature, and those effects would still require a legal target to have it explore.)