Why does it say I can only order one phone per order for a specific carrier?

Holliey wrote:

I tried to order two phones both for the service provider Verizon.

So presumably you are ordering two iPhone XRs to be used with Verizon, each iPhone, of course, having its own telephone number and setup.

So once you start the ordering process for the first iPhone, you see a screen like this:

Then you choose a response and continue. Please explain what happens from then forward.

I think the short answer to your question is you can in fact order two iPhone XRs using Verizon, each as a separate order with its own telephone number (and subsequently set up each with its own Apple ID), and each separately invoiced and (perhaps) shipped.

Holliey wrote:

Why ...

You are asking us to guess about everything related to your specific circumstances. We are volunteers. We'll be glad to help answer your questions, but you must provide us with all relevant facts. If you don't know whether a fact might be relevant or not, then include it.

Glad that helped.

Enjoy your new iPhones!