Whenever a post or page is created, WordPress will create a unique ID.
In cases where you need to get the ID, open your WordPress admin/Dashboard and click the relevant Post or Page button to display a listing of your posts or pages.
In your post or page listing, hover your mouse over the post or page title and the status bar (bottom left corner) of your browser will display their respective IDs as shown in the example below:

In case you do not see the status bar at the bottom of …