what is the maximum number of people to fill out a single questionnaire?

@zhanghuaizhanghuai I believe the Qualtrics free account is limited to 500 responses per survey.  You could possibly work around this by creating separate surveys. 

You can export data and combine responses external to Qualtrics, as not sure the free account enables you to merge responses across surveys -so you may need to be a little more creative in how you report/analyse the data if doing it across multiple surveys - but it’s feasible.