The number of core licenses needed depends on whether customers are licensing the physical server or individual virtual operating system environments (OSEs).
Under the Per Core licensing model, each server running SQL Server 2014 software or any of its components (such as Reporting Services or Integration Services) must be assigned an appropriate number of SQL Server 2014 core licenses. Microsoft SQL Server 2014 Core-Based Licensing
Note: for customers who upgrade to SQL Server 2014, a 20 core limit applies to the software. However, customers with active Software Assurance (SA)coverage can continue to renew SA on Enterprise Edition servers and upgrade to SQL Server 2014 software. Special Note for Enterprise Edition Users: With the introduction of SQL Server 2012, Enterprise Edition was removed from the Server + CAL mode and new server licenses are no longer available. A minimum of 4 core licenses is required for each physical processor on the server. Determining the number of licenses needed is done by multiplying the total number of physical cores by the appropriate core factor found in the core factor table.
In summary: download the SQL Server ISO found in the Volume License Center under System Center 2012 Configuration Manager, and run the installer on a clean server.
I found a way around though, wrote it up here: SCCM's SQL Stuck in Eval mode? How to fix! It will instead revert to displaying the 'Choose Evaluation or enter key screen'. However, if you try to run this installer side-by-side on a server with a pre-existing install, SQL Server Setup will detect the other instance of SQL and not actually display the key. If you use the ISO that you download from your volume license page, the SQL Server ISO should include the 'free SCCM' license baked in. I found the answer, for anyone else stuck in the same problem.