From whales to a frog the size of a fingernail, always two eyes.
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The real answer: common ancestry. Evolution tends to keep things that work, and animals with 2 eyes share a common ancestor with a species that also had 2.
The more nuanced question might then be: why did that species have two, and why was that aspect near-universally maintained in descendant species? That comes down to a few parameters:
– chance of evolution can’t be ruled out
– body plans tend to favor symmetry, and animals with bilateral symmetry will have most things in pairs
– two is the minimum necessary to get to what is roughly/nearly 360 degrees (for prey species) or stereoscopic/binocular vision (for predator species)
– eyes are sensitive, fragile organs -more of them means more vulnerable spots to protect, so minimizing them to only what you really need makes biological sense
How much each of these factors played is difficult to determine.
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