Managing global labels is done via the Apps menu on your Jira Cloud settings. Once the app has been installed, a new menu item called Manage Labels will appear. Click on it to get to the Manage Jira Labels page.
Checking Label Use on Your Jira Instance​
Go to the global Manage Labels in your Jira instance. You should see something like this:
This is a list of all the Jira labels used in your instance. The second column shows you how many times a label has been used. Clicking on that link opens a new tab with a filter that shows all issues with that label in your project.
INFO
If a label is used on an issue that you do not have access to, it will not show up here and will not be counted.