For the free version of Mentimeter, the maximum number of questions you can create is 2, and 5 Quiz questions.
If you wish to get an extra question for your free account, simply send an invitation to a friend to get them to join Mentimeter as well! This option will show up when you try to add your third question.
Should you wish to get access to unlimited number of questions, you will need to purchase an upgrade instead. Do visit our pricing page to find a plan that suits you!