How do I find a lost Ipad Pro with its serial number?

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPad has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad have been lost.

You don't, sorry.

These are the only steps you can perform in such a situation:--> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Regards

Giulio