How can I get IMEI number for my stolen iPhone?

IMEI number is written on the bottom of the iPhone box if you still have it, and if you have another apple device like another iPhone, an iPad or a Mac you can go to settings>your name(Apple ID) scroll down and you will see all your devices the iPhone stolen included, tap on it and you will see the IMEI number (unless you removed it from your account)

Having the IMEI isn't going to help you very much. You should contact your carrier and let them know the phone has been stolen. They will also have the IMEI. You may also be able to see it if you log into your account on your carrier's website or app.

See also:

Find your lost iPhone or iPad with Find My - Apple Support

The IMEI number and a valid purchase receipt is needed for an insurance company claim, and for a police report number, usually they also ask you to call the carrier and blacklist the IMEI. Unless you have an Apple Care + with loss and theft

Your phone company (carrier) might know your IMEI number.