is my iphone X sim card compatible with iPhone 12

Your iPhone X SIM card does fit in your iPhone 12.

Learn which size SIM card your iPhone or iPad uses - Apple Support

However, I can’t imagine why you would choose to use it.

Your SIM card contains no data that the Apple ecosystem uses. And using an old SIM would certainly prohibit you from using new cellular carrier features such as 5G.

Yes, see: Learn which size SIM card your iPhone or iPad uses - Apple Support

Assuming your new phone is for the same cellular provider, or unlocked, yes. They both use a Nano sim.