It's interesting that neither the w3, mozilla, nor the spec, itself, specifically mentions that there can be only one:
https://www.w3schools.com/tags/tag_caption.asp
https://developer.mozilla.org/en-US/...lement/caption
https://html.spec.whatwg.org/multipa...aption-element
Reading the text closely enough implies it, but it would have been nice if they'd have mentioned it. Good catch.
Of course, it looks like you can put all kinds of formatting in the <caption></caption> pair.