Microsoft Dynamics AX 2012 R3 CU12 is now available. Cumulative Update 12 build number or version is 6.3.5000.138. You can check this by clicking on about dynamics ax button.
Microsoft official KB document Number is KB3199741.
To learn more about known issues in CU 12, You need to log int o LCS by using Customer Source or PartnerSource account then search with CU12 Known issue.
You can also find on Microsoft partner source for pdf document WhatsNewinAX2012R3CU12.pdf to get details that whats new in CU12.