You can set up the numbers of frames in your sequence when you first create a sequence.
There are 2 ways to add new frames into the sequence, once a sequence is created.
1. Check the toolbar, and tap on the "add frame" button to add new frames. If you cannot see this button in the toolbar, please switch the toolbar by tapping the button at the right or left side.
2. Tap on the "add frame" button on the shortcut toolbar to add new frames.