Hi
First advice; To be sure I would call the licensing for the Action Pack's coverage.
My advice, those CAL are for the OS itselft to cover lab usage, not for direct production's use.
Per the documentation;
Benefits include the latest internal-use software that you can use to run your business, sell your solutions, develop applications, and train your employees. These licenses must not be resold or used for direct revenue-generating activities, commercial purposes, personal purposes, or customer training.
Core Benefits now give partners the power of choice to mix-and-match online services and on-premise software licenses. Partners can choose between work-alike solutions for productivity, demonstration, development, testing, and internal training purposes.
Refer to the Product Usage Guide for important details regarding license use rights and limitations about each software product or online service. Your organization must comply with the use terms outlined in the Product Usage Guide.
To cover your subquestion, the licensing is that you must cover your user with a CAL for the use of the systems but it's only a one shot coverage, ie; your 15 user need 15 CALS only, even if 2 or more Windows Server are running. The important point is you need CAL at the same version as your Windows Server, ie; you are not legal if you install Windows Server 2022, but your user cal are Windows 2016's one in example. The contrary work as you have downright right. (Windows Server 2016 used but user call in 2022).
Just a reminder, we dont talk it, but for the OS now you need to cover the CPU core per physical host to be in legality. In example if you have 2 Windows Server but that can be run in a hypervisor cluster on two host, you need them (physical's host) licensed twice.
Thanks, I hope I cover your question correctly.