i want to know my imei number my phone is lost

There no way to find a lost iPhone using its IMEI number.

The only steps you can perform in such a situation are provided in this support article:--> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support, and/or in this one:--> Use the Find My app to locate a missing device or item - Apple Support

As useful as it may be, these are the instructions to find the IMEI number of your iPhone: -> Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio

If you have another iphone or ipad then in Settings, [your name], tap on the device to see that data.

Or there's the box it came in.

Learn more about how use the Find My App\

Find My - Official Apple Support

Then look at this Apple article and my additional advice below it:

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

Tracking lost or stolen phones

The 'Find My' tracking is to help you discover if you left your phone in the office or the restaurant. Lost or Stolen phones are rarely ever seen again.

Risk mitigation

Get your provider to

Cancel your SIM.

Send you a replacement SIM on same number.

You can then put the SIM in another iPhone to get the codes you will need.

Phishing warning

Its possible for a thief to obtain your phone number particularly if you did not have a PIN set on the SIM. So do not respond to texts to sign into your account! They are NOT from Apple.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Resign to the fact that if you had a passcode set and you do not provide your Apple ID via phishing texts then your data is safe but you may never see the phone again.