20 is the cap for the number of HTML, image, and video assets per ad group for an app campaign.
The correct answer is: 20.
App campaigns rely on creative rotation powered by machine learning to choose the right creative for the right inventory, the right user, and the right moment. Follow these recommendations for the best results:
Use diverse creatives in content, theme, length, and orientation.
Upload more creatives; use all 20 slots if you can.