// Nlog version: 6.0.3

 // Nlog config:
 NLog.LogManager.Setup().LoadConfiguration(builder =>
 {
     builder 
     .ForLogger(loggerNamePattern: "*") 
     .FilterMinLevel(LogLevel.Trace) 
     .WriteToFile(fileName:  "${basedir}/logs/test.log",
                  archiveAboveSize: 1024 *10,
                  maxArchiveFiles: 10
                  );
 });

  // Logger
 var logger_test = LogManager.GetLogger("abc");
 
 // Loop test
 string msg = string.Concat(Enumerable.Repeat("ABCDEF", 200));
 int loop = 0;
 while (true)
 {
     Console.WriteLine("press any key to log...");
     Console.ReadKey(true);
 
     Parallel.For(0, 1000, i =>
     { 
         // to log
         logger_test.Trace(msg); 
     });
     Console.WriteLine($"loop-{++loop} completed");
 }

   loop-1 completed:
            file list(10):
                test_91.log ... test99.log,
                test_111.log -----Q: Why is it 111? Why not 100?

   loop-2 completed:  
            file list(10):
                test_91.log ... test99.log,
                test_222.log -----Q: Why is it 222? And why not delete the older log-files?