It is 4 cores per task. If you were able to run it with 16 cores, you were probably using HPC licenses.. If you are not able to run it with higher cores now, the HPC licenses could have expired. You can check with your license server admin.
Refer to included cores in this doc - https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v232/en/licensing/HPC_detail.html?q=base%20cores
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