Multiple processes are multi-threaded in parallel?

[hypnosis01]

public class AspSample : StepBody
{
private static readonly Logger log = LogManager.GetCurrentClassLogger();
private static volatile int _counter = 0;

public AspSample()
{
    log.Debug("AspSample init ...");
}

public override ExecutionResult Run(IStepExecutionContext context)
{
    if (_counter % 2 == 0) 
    {
        Thread.Sleep(2000);
    }
    log.Debug($"StepBody: {nameof(AspSample)}");
    AspSample._counter++;
    return ExecutionResult.Next();
}

}

var host = serviceProvider.GetService();
host.StartWorkflow("PTWorkFlow", 1, null);
host.StartWorkflow("PTWorkFlow", 1, null);
host.StartWorkflow("PTWorkFlow", 1, null);
host.StartWorkflow("PTWorkFlow", 1, null);

Multiple process implementations were initiated。

Multiple processes are executed serially.......

[hypnosis01]

How do I make each process instance execute in a separate thread?