GitHub seems has no predefined environment variables or contexts to get the number of cores.
As a workaround, you can run the following commands on each OS to view the number of cores:
WMIC CPU Get DeviceID,NumberOfCores,NumberOfLogicalProcessors
WMIC CPU Get /Format:List
lscpu | egrep 'Model name|Socket|Thread|NUMA|CPU\(s\)'
Then you can try to get the number from the output of the command, and set an output parameter with using the number as value.
In a workflow, The output parameter only can be used between the steps in a same job via the syntax steps.<step_ID>.outputs.<output_name>. If you want to share the output parameter in other jobs, you can try to use artifact to pass the output parameter across jobs.
My reply in this ticket shows a simaple demo about how to pass variables/parameters acress jobs.
Hope this can help you.