As I understand it, all users that are managed with an Azure AD Premium paid feature must be licenced, so in this example, all users that will be managed with conditional access require Azure AD Premium licences.
This makes it much clearer though like any licence topic its best to contact Microsoft directly or a partner, for example, to be certain:
"Although obtaining a subscription is all you need to configure paid capabilities, you must still assign user licenses for paid Azure AD paid features. Any user who should have access to, or who is managed through, an Azure AD paid feature must be assigned a license. License assignment is a mapping between a user and a purchased service, such as Azure AD Premium, Basic, or Enterprise Mobility + Security."
Quickstart: License users in Azure Active Directory
