The maximum number of questions that a Question Drafter or Question Submitter can ask at once using the Import questions feature is 500 questions. If more questions need to be asked, a new template can be extracted to add more questions.
There is no maximum amount of total questions in Q&A. The maximum amount of total questions in Q&A depends on the question limit set by the Vault Admin. If no limit is set, there is no maximum amount of questions. Visit Question limits, to learn more about this feature.