Some spells can have multiple targets, like eldritch blast (at higher levels) or scorching ray.
Eldritch blast (PHB, pg. 237) says:
The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level. You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.
Scorching ray (PHB, pg. 273) says:
You create three rays of fire and hurl them at targets within range. You can hurl them at one target or several.
Make a ranged spell attack for each ray. On a hit, the target takes 2d6 fire damage.
Let's say I ready a spell with multiple targets and that my trigger is something like:
"I attack the orcs (plural) when I can see them"
(Let's assume that my character knows there are orcs hiding behind something and will jump out any moment now; if we need to explain why my PC would know this, perhaps because an ally will do something to draw them out of hiding before the orcs' and my PC's next turn, or perhaps I just keep readying eldritch blast until they finally do come out.)
Given that you can choose not to respond to a trigger for readied actions (PHB, pg. 193):
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.
...then if I ignored the first enemy that appeared but responded to the second, could I target both enemies as I "release" the spell, or would I only be able to target the creature that is the trigger I'm responding to (meaning the second orc in this case)? Or is the trigger not specific enough to be a valid trigger, rendering this whole question moot?