Does the iPhone 11 card holder fit in the 12
Does the iPhone 12s SIM card holder fit in the 11?
iPhone 12
Question marked as
No. They are different sizes and slightly different shapes. You should never attempt to use a SIM card tray form a different iPhone, as they can get stuck and damage the slot.
Use the sim tray from the iPhone 11 to insert a SIM card into it.
If you lost the tray, then you will need to contact Apple support for a new one.
Link->Remove or switch the SIM card in your iPhone - Apple Support