Hello Ryan Cheng
Neural models training count is reset every calendar month. Increasing the quota for custom neural model training from 20 to 40 per calendar month will likely incur additional charges as billing method is per hour.
Open a support request to increase the monthly training limit.
Please refer to this service limits documentation for more details: https://learn.microsoft.com/en-us/azure/ai-services/document-intelligence/service-limits?view=doc-intel-4.0.0#custom-model-usage