If you put one at the end of a sentence, make sure we’re really asking a question. And if we don’t provide the answer (either in the content or with a CTA), make sure it’s a rhetorical question. Why? A question the reader can’t answer comes off as condescending. Not our style.
Questions work well in subject lines, headlines and confirmations. Avoid putting them where interaction is harder to come by, like an empty state or tooltip.
Overall, question marks should be used sparingly. However, if the content is formatted with question, answer, question, answer, etc., multiple is totally fine.