Apple's upcoming iPhone 12 could feature 4K 120 fps and 4K 240 fps video recording, according to code unearthed in recent iOS 14 beta builds. This feature is likely to be restricted to the iPhone 12 Pro and iPhone 12 Pro Max models. There's also a possibility that the iPhone would record natively at 4K 120 fps and then interlace the video to 240 fps.
https://www.notebookcheck.net/iOS-14-code-points-to-support-for-4K-120-and-4k-240-fps-video-in-the-iPhone-12-Pro-and-Pro-Max.478171.0.html