My iphone x has been stolen the day before yesterday, how can i get the imei number of that phone and want to block that device ?

Good advise from previous posting

To Add, if one wants the IMEI Number Log into My Apple ID and look for the iPhone in Devices.

Just verified before posting here and my iPhone showing both Serial Number and IMEI Number

It may be on the invoice/receipt from when you purchased the iPhone. Also, contact your carrier - they should be able to block the phone even if you don't have the IMEI (as long as you can prove you are the owner).

If you had your phone set up with Find My you can erase the phone via your iCloud account.

Also, follow these instructions -> If your iPhone, iPad, or iPod touch is lost or stolen.

You are welcome and thank you too.

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional question may arise

The previous persons posting should also be followed .