DoubleTake by FiLMiC Pro captures multiple iPhone camera footage simultaneously
apps.apple.comI thought they were going to support recording all cameras at once (IIRC that was demoed at the iPhone 11 launch).
From what I can glean of this Verge article [0], it seems two cameras is the maximum possible at this time.
[0] https://www.theverge.com/2020/1/28/21083583/filmic-doubletak...
I take no ownership of this but found it an insanely cool free app that shoots a single video with multiple cameras.