@Hugo Leyton Object limit defines how many objects can be created in the directory including Users, Groups, Devices, Applications, Service Principals objects etc. it doesn't mean that there is a limit of 500,000 logins for your applications. There is no limit for logins.
The object limit for Azure AD Free version is by default 50,000. If you add a custom domain to your Azure AD tenant, this limit is extended to 300,000 automatically. In order to further raise this limit to 500,000, you have to open a support ticket and request for extension.
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