License question

• A shared mailbox requires a Defender license only if it benefits from Defender protections (e.g., Safe Links, Safe Attachments).
• If the shared mailbox is unused and is excluded from any Defender policy scope, then no Defender license is necessary.
• If you want the shared mailbox to be included in Defender protections, then it must be licensed.

“For Microsoft Defender for Office 365 … licenses must be acquired for users or mailboxes falling under one or more of the following scenarios: … Shared mailboxes that benefit from Defender for Office 365 protections.” 

Source: https://docs.microsoft.com/en-us/office365/servicedescriptions/office-365-advanced-threat-protection-service-description#licensing-terms

So, in your case (10 licensed users + 1 shared mailbox), you only need those 10 Defender licenses provided the shared mailbox is not covered by Defender protections.
If you decide to include the shared mailbox under Defender policies, then it would need its own license.

Hope it's clear for you Rafa.

Thanks