will the sim card from my iPhone 6 work in the i phone 12, 12 pro, or 12 promax?

Yes. More or less. If you want 5G service, you will probably have to get a new SIM from your carrier.

The Nano Sim is same size as newer iphone it suppose to work. I using Nano Sim card from iPhone 6 to my new iPhone 12 Pro Max, no issues. But if you thinking about 5G service, you have to contact your wireless carrier about it.

Yes. nano-SIM is same size in all iPhones 5 and above.