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.
References:
https://docs.microsoft.com/en-us/azure/sql-database/sql-database-managed-instance-resource-limits#obtaining-a-larger-quota-for-sql-managed-instance