The purpose of your startup pitch is to simulate the interest. It is not there to cover all aspects of your startup and burden the audience with too much information. So basically pitching the idea to your audience is to generate enough interest to get into the second meeting.
Hence I recommended 10 slides as shown below. I know you must be thinking that its impossible to pitch the whole idea within only 10 slides. But this low number of slides forces you to concentrate on absolute essentials. You can add few more but you should never go beyond 15 slides.