Step-By-Step: Implementing Azure AD Password Protection On-Premises

Hi juangonzalez,
here the link related to the licensing of Azure AD Password Protection:
https://docs.microsoft.com/en-us/azure/active-directory/authentication/concept-password-ban-bad
Please pay attention to the note:

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On-premises AD DS users that aren't synchronized to Azure AD also benefit from Azure AD Password Protection based on existing licensing for synchronized users.
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So let me translate this in your case, if you have 1000 users, 100 are synchronized with Azure and 900 are only local AD Users, you should have 100 licenses AD Premium P1 or P2 to provide AAD Password protection with Custom password list to this 100 users on the cloud side but also the 1000 users on-premises can benefit of the Azure AD Password Protection.
Remember in this 100 user you should have a Global Admin or buy 101 licenses.
Many thanks for your question 😉

Daniele De Angelis