I have a new Apple Watch 7 titanium with gps and cellular I have added a new line for the watch but how will I receive messages from my iPhone with a different phone number

Your watch uses your phone's number via an Esim built into the watch;

Technically, the cellular Apple Watch has its own phone number assigned to it but in practice it takes on the identity of the phone number of the iPhone you set it up with. Each carrier calls it something different. Basically what this service does is it allows you to make and receive phone calls and text messages from your Apple Watch (even when your iPhone is not nearby). I guess the simplest way to describe it is that it treats your Apple Watch like it is a 2nd iPhone that has the same phone number as your iPhone.

So if you leave your iPhone at home and only have your Apple Watch and you call or text someone (or they you) they see the phone call as coming from you (your normal iPhone cell number or text number). It truly “syncs” the phone number associated with your iPhone with your Apple Watch.

On your cell phone bill you will see your Apple Watch listed as a separate phone number (one that is unique to your Apple Watch). This number associated with your cellular Apple Watch has no purpose other than as a billing identifier.