How can i find my old iphone 11 imei number

Having the IMEI number will not help you unless you are using some unofficial app or website

If you have another iPhone or iPad use the Find My app, or on any web browser go to https://icloud.com/find

and look for it there

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. They are often processed invisibly by organised crime gangs.

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

It’s 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 askimg you to sign into your account to get your phone back! 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 inaccessible by anyone else but you may never see the phone again.

Phone marked as Lost - unmark lost phone

Use Lost Mode in Find Devices on iCloud.com – Apple Support (UK)