To find a categories ID, you need to logged in and go to your dashboard. Open Posts – Categories, and click on one of the category names. The url of the page you are on will show what the category id is. For example, in URL below, your Category ID is 13.

https://www.webminimalist.com/wp-admin/edit-tags.php?action=edit&taxonomy=category&tag_ID=13&post_type=post