Login to Azure or Launch Azure portal.

Head over to Azure Active Directory.

Select Devices.

Device Settings.

Set it to selected users.

Require Multi Factor Auth if you have set this up.

Note*

Users may register devices with Azure AD is required for 365 registration and Microsoft Intune enrollment. If you have configured these the setting will be unavailable to change.

There is another setting below that lets you selected maximum amount of devices joined per user or unlimited.