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There is no hard limit for the guest user invitation. However, there is a limit for Azure AD itself.
Azure AD B2B is subject to Azure AD service directory limits.
By default, a maximum of 50,000 Azure AD resources can be created in a single tenant by users of the Azure Active Directory Free edition. If you have at least one verified domain, the default Azure AD service quota for your organization is extended to 300,000 Azure AD resources.
Resource limitations apply to all directory objects in a given Azure AD tenant, including users, groups, applications, and service principals.
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