Customize Branding Licenses

Hi

I wanted to ask how does the feature of Azure AD Premium is calculated for users?

I have Office 365 for 500 users

I want the AD Premium feature to customize the login page

when I add AD Premium license for 1 user I can access to the customize branding and modify the access page.

but what does it mean in terms of licensing? 

obviously, the URL is available for all users no matter what and I don't think only the licensed user will be able to login to Office 365 :/

would be happy to get an official answer

thanks

Tamir Levy