Hi @Techit Sriwichai ,
Thanks for the post! Defender for Business Servers is meant for up to 300 employees and no more. The licensing agreement requires that you have no more than 300.
After that you would need to upgrade to the Microsoft 365 Business Premium subscription, or consider using other Enterprise solutions such as Defender for Endpoint, Microsoft 365 Defender, or Microsoft 365 for enterprise.
Reference: FAQ
It's always free to speak with a representative from the licensing/subscription/sales team though, so if you have further questions I would recommend speaking with them.
You can reach them via phone or chat: https://azure.microsoft.com/en-us/contact/#contact-sales
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