Reference:
https://docs.microsoft.com/en-us/azure/sql-database/sql-database-managed-instance-resource-limits#obtaining-a-larger-quota-for-sql-managed-instance Many Azure resource have quote limits. The purpose of the quota limits is to help you control your Azure costs. However, it is common to require an increase to the default quota.
You can request a quota limit increase by opening a support request. In the support request, select 'Service and subscription limits (quotas)' for the Issue type, select your subscription and the service you want to increase the quota for. For this question, you would select 'SQL Database Managed Instance' as the quote type.
https://docs.microsoft.com/en-us/azure/sql-database/sql-database-managed-instance-resource-limits#obtaining-a-larger-quota-for-sql-managed-instance

To migrate to Azure and decommission the on-premises data center, you would need to create the 5,000 user accounts in Azure Active Directory. The easy way to do this is to sync all the Active Directory user accounts to Azure Active Directory (Azure AD). You can even sync their passwords to further minimize the impact on users.
The tool you would use to sync the accounts is Azure AD Connect. The Azure Active Directory Connect synchronization services (Azure AD Connect sync) is a main component of Azure AD Connect. It takes care of all the operations that are related to synchronize identity data between your on-premises environment and Azure AD.
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-whatis