For example in a résumé, are
Experience in a couple of rendering tools
Experience in some rendering tools
the same from the point of view of formality?
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I would suggest neither are correct in formal writing. As commentors have already pointed out, in this instance 'a couple' would refer to literally two items. However, 'some' also feels clumsy to me in that sentence.
I would suggest using one of the following in a formal document, such as a resume:
Experience in a number of rendering tools.
Experience with various rendering tools.
Experience in numerous rendering tools.
Experience in a multitude of rendering tools.
I have ordered these in roughly ascending order of number, with "a number of" being the least, and "a multitude of" being the most, depending on how many tools you wish to express your experience with.
I might also suggest changing the word "in" to "with" in any of these sentences.