Hello Dear,
Thank you for your question and reaching out.
As per your Query,
Let me explain you with An example:
An RDS CAL is a license that allows users/devices to access and utilize Windows Server published resources, such as Windows applications, desktops and files. This license model provides a way to pay for capacity used: the more users or devices accessing your resources, the more RDS CALs you require
So, with the number of users you have, You need same number of RDS CAL license.
so this means that, For 15 users, You will need same number of RDS CAL license's.
Please follow the helpful Links:
https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/rds-client-access-license
https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/rds-install-cals
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