How much video can you record on a 64 GB iPhone 12?
The Camera settings on iPhone 12 shows

4K @ 60fps is 440MB per minute.
4K @ 30fps is 190MB per minute.

I don’t have an iPhone 12 to know exactly how much space is reserved by iOS but supposing it’s 15 GB that leaves 110 minutes of 4k60 or 257 minutes of 4k30.