Thanks for posting your question in the Microsoft Q&A forum.
If all users that have access to the shared mailboxes have a Microsoft Defender for Office 365 license, then the shared mailboxes will be automatically protected.
This means if all 55 of your users have a license for Microsoft Defender for Office 365 Plan 1, and these users are the ones accessing the 10 shared mailboxes, then the shared mailboxes should be protected without requiring additional licenses specifically for the mailboxes.
However, if you were to have a user without Defender for Office 365 who needs to access a shared mailbox, that mailbox would not be protected by Defender features until that user is also licensed.
You can refer to this link for details:https://zcusa.951200.xyz/en-us/office365/servicedescriptions/office-365-advanced-threat-protection-service-description?source=recommendations#licensing-terms
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