I have lost my iPhone. Should I mark it as lost or erase iPhone and can apple track via IMEI number?

This document has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

- The Find My service is the only recovery feature Apple offers. It MUST be set up before the device is lost. If you did not do that it is too late to do it now and there is no other feature Apple offers that can help you find it.

It can not be tracked via the IMEI. The only way to track it is using Find My.