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

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

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