I have just bought the new iPhone 13 pro Max, Just found out, one sim card only, can u change to the 2 SIM card model?

The standard iPhone supports one physical SIM and one eSIM.

Only phones sold in mainland China and Hong Kong supports dual physical SIMs.

Given the fact that you received a single physical-SIM device, I have to assume you're not in China, therefore your options are limited.

Most carriers, though, support eSIM and are able to issue an eSIM instead of a physical SIM, so check with your service provider.

Yes. That's it. No US-based iPhone has two physical SIM slots.

The China one has two slots because eSIMs are not supported by the Chinese authorities. They also use different cellular frequency bands, so may have issues working outside of China.

The list of supported bands by iPhone model is available at: iPhone - Supported 5G and LTE networks - Apple