CAL is a Microsoft licensing construct that stands for client access license. In practice, purchasing CALs allows individuals to access commercially available software purchased by their organization.
The use of CALs has been expanded to include other licensing approaches, such as per-system licensing.
While a CAL for software such as Windows Server may be relatively inexpensive per user, there are other factors that contribute to the true cost of software in addition to CALs.
To continue with the Active Directory example, one important factor that influences its cost is that it is entirely on-premises.
That means that in order to use AD properly, you must not only pay the CAL per user but also have physical servers to house your AD instance, as well as Windows Server licensing. Other costs, such as systems and infrastructure, are added on top of that.
Do I need user CALs for every server?
Yes, you will require some kind of Windows Cals to gain access. A User CAL is purchased for each user who connects to the server to use services such as file storage or printing, regardless of the number of devices they use.