How can I track my stolen iPhone 12?

nothing apple can be tracking using a serial number.

about IMEI tracking then if your local law enforcements present a warrant to the carriers in your country they can track it using the IMEI, it's a rather costly procedure so in most countries it's only ever used in case of missing persons cases.

so if your phone was stolen or lost then it's somewhat unlikely that your law enforcements will put that in motion

but you can try to contact them.

Hello,

Your options are included in this link:

If your iPhone, iPad, or iPod touch is lost or stolen

Good luck.