A client needs a Windows Server set up.
The Server will run Exchange.
The client will have a max of 10 employees.
At most one user will ever need to remote-desktop in.
How many User CALs will need to be purchased and what kind?
Should there be 10 User CALs for the Windows Server, 10 User CALs for Exchange and 1 RDS CAL?

Some versions of Server come with CALs. So there is no clear clean answer. And this:

Remote Desktop supports two concurrent connections to remotely administer a computer. You do not need a license server for these connections

So there's that. So check what CALs your version comes with and try simple. Like 10 minus what CALs the server came with.

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