How I can I find my iPhone X it’s offline but I know my serial number

If the phone is offline, you can't track it. Knowing your serial number won't help you.

The only way to track a missing phone is all three of the following must be true:

1. Find My was turned on
2. The phone must be powered on
3. The phone must be connected to a network

If any of the above 3 are not true, you can't track the phone. You have to look for it the old fashioned way, which is to look for it.

See this link for help with a lost or stolen iPhone --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

The phone must be turned on and be online in order to send a signal showing its location. The serial number will help only if you report the loss to the police or a finder needs verification that you are the owner.

Fafanealexis wrote:

How I can I find my iPhone X it’s offline but I know my serial number

If you lost your phone, you find it the old fashion way, you physically look for it, since it is offline. Serial number won't help you track it.