SQL 2012 Enterprise is only licensed by Core. If you think you are using CAL licenses with EE then you may not be correctly licensed.
SQL 2013 BI and SQL 2012 Standard can be licensed by CAL, but with Enterprise the only valid method is by Core. It may well be the same situation with SQL 2014.
If you are upgrading from Standard or BI to Enterprise then there is a simple process (see BOL) where you enter your new license key and restart SQL Server.
If you are already licensed by Core and have licences for fewer cores than are visible on Windows then you should use Processor Affinity within SQL Server to remain within your license terms. If you buy more core licences you only have to change your processor affinity to take advantage of the new licences.
If you are not certain of your license situation then my advice is to contact your license reseller.
Original author: SQL Server FineBuild 1-click install and best practice configuration of SQL Server 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005. 1 Dec 2016
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Disclaimer: All information provided is a personal opinion that may not match reality.
Quote: "When I give food to the poor they call me a saint. When I ask why they are poor they call me a communist." - Archbishop Hélder Câmara