Do administrators need a Client Access License (CAL) to access a Windows Server 2012?

No, administrators do not need CALs to access a Windows Server 2012. However, this scenario is limited to two users or two devices.

Source: Official Microsoft Volume License Blog, https://blogs.technet.microsoft.com/volume-licensing/2014/03/10/licensing-how-to-when-do-i-need-a-client-access-license-cal/ (accessed 08.03.2015)