Times for presentations can range from 30-50 minutes, but the vast majority are 45 minutes followed by 5-10 minutes of Q&A.
If your presentation is supposed to be 20 minutes long, getting through 45 slides is almost impossible. Also remember that slides with lots of builds take more time than a slide with no builds. A good rule of thumb for most people is 1 slide for every 3 minutes of presentation. The best thing to do is time yourself in advance!
Make sure your slides are readable. In general, nothing below 18pt type is going to be readable by anyone not in the first few rows. 24pt and up is usually safe. Best recommendation – use images, be bold, be clear, be recognized.