Batch render with all cores in a Mac / Autodesk Maya
First, you need get information about the numbers of core of your mac, so, go to the imac icon.
Later, go and click in more info.
Now, you could see the characteristics of your Mac, the information important here is the Total Number of Cores (in this case 4).
Continue, open your Autodesk Maya, go and click in Render - Batch Render - options.
Later, Deselect the Auto Render Threads and manually type the number of cores that you saw above.
Go to Windows - Settings/Preferences - Preferences
Now, you only need to click in save, this steep Is important for the changes to take effect.
Finally, Go to Render - Batch Render and enjoy of rendering with all your cores.