public static int ThreadIndexCount { get; }

The job system creates a number of job worker threads no greater than the number of logical CPU cores for the platform. However, because arbitrary threads can execute jobs via work stealing, the job system allocates extra workers which act as temporary job worker threads. JobsUtility.ThreadIndexCount represents the maximum number of job worker threads plus the temporary workers that the job system uses. As such, this value is useful to allocate buffers which should be subdivided evenly between job workers because JobsUtility.ThreadIndex and NativeSetThreadIndex never return a value greater than JobsUtility.ThreadIndexCount.