my question has always been why (I think most vertebrates) stop at two? It seems that an extra eye here and there could be really helpful. Maybe it's because all verterbrates evolved from an ancestor that had two eyes, and once the template is in place, it was simply too deep a local maximum to evolve out of? Similar to the 5-digit hand design that all vertebrates share.

Well, from what I've read...

Spiders have 8 eyes. As with vertebrates, this number doesn't change, but there is variation in what it means.

A "normal" spider doesn't really use its eyes. It just has them.

Some spiders are different and rely on their vision. Those spiders have two primary eyes, which they rely on, and six secondary ones, which they don't.

Moving to insects, they often have compound eyes. Two compound eyes. A mantis has two primary compound eyes and three secondary non-compound eyes.

All this convergence suggests to me that even if you have the option to grow more eyes, the correct number is two.