Can a broken Sim tray on iPhone 12 be replaced?

It can be. Apple does not sell replacements, so you may need look for one from a used damaged phone on ebay or the like. It needs to be from an identical mode though. You cannot use an iPhone 12 Pro sim tray or an iPhone 13 sim tray in an iPhone 12.

You can also try contacting Apple Support directly, and see if they can provide one.

click here ➜ https://getsupport.apple.com/

SIM Card Tray Slot for iPhone 12 Pro/12 Pro Max - Black - Incl Rubber Ring | Apple iPhone 12 Pro Max | iPhone | Mobile Spare Parts | MMOBIEL

Amazon.com: SIM Card Slot Tray Holder Replacement Compatible with iPhone 12 Mini Sim Card Tray (Blue) : Cell Phones & Accessories

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Sim tray for IPhone 12 - Google-