How i add esim for iphone 12 pro max

If you have an option for eSIM on your device then you will have EID. Please check if you have it.

Settings > General > About > EID (See the pic below)

eSIM on iPhone is not offered in China mainland. In Hong Kong and Macao, iPhone 13 mini, iPhone 12 mini, iPhone SE (2nd and 3rd generation), and iPhone XS feature eSIM. Learn about using Dual SIM with two nano-SIM cards in China mainland, Hong Kong, and Macao.

You can activate your eSIM while you're setting up your iPhone if your carrier supports eSIM Carrier Activation or eSIM Quick Transfer. With eSIM Carrier Activation, your carrier assigns an eSIM to your iPhone when you purchase it. With eSIM Quick Transfer, you transfer the SIM from your previous iPhone to your new iPhone without contacting your carrier. With either method, to activate your eSIM during setup, turn on your iPhone and follow the instructions.

See this link for further reference

About eSIM on iPhone - Apple Support