From whales to a frog the size of a fingernail, always two eyes.
In: 5
One, you’re at risk of losing it, which’ll leave you blind.
Two, you get binocular vision, which helps a lot to judge distances to objects.
Three would be a minor improvement, but it would cost more in terms of space. Put one on the back of the head and you’d be safer, but…. And most animals start with a bilateral symmetry, so adding a third would be difficult, as an evolutionary step.
Four, you’re probably past the point of diminishing returns. Devoting twice as much space in your head to vision would mean giving something else up.
More, well, having many simple eyes spread around your head plus a brain set up to synthesize their inputs might work.
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