The script operates as follows.  Assume that we have a logfile called, "mylog.txt".  I want to keep the last five copies as well as the current logfile.  The files will be rotated with the following convention:

mylog.txt -> mylog1.txt
mylog1.txt -> mylog2.txt
mylog2.txt -> mylog3.txt
mylog3.txt -> mylog4.txt
mylog4.txt -> mylog5.txt
mylog5.txt -> <delete>
 

I have included a couple of function to provide a working sample, main, print, MakeFile.  The functions that are needed are Archive and GetFileName.

var fso = new ActiveXObject( "Scripting.FileSystemObject" );

// Function to demo archiving
function main()
{
 for( k = 0; k < 10; k++ )
 {
  print( "Current Loop: " + k );
  Archive( "mytest.txt", 5 );
  MakeFile( "mytest.txt" );
  
  WScript.Sleep( 1000 * 31 );
 }
}
main();

// Archive - handles file rotation
function Archive( srcfile, retention )
{
 // Find the next open ID for file rename
 var NextID = 0;
 for( i = 1; i <= retention; i++ )
 {
  var fname = GetFileName( srcfile, i );
  if( !fso.FileExists( fname ) )
  {
   NextID = i;
   break;
  }
 }

 // If we did not detect a "free" ID to use, we are at max.
 if( NextID == 0 )
  NextID = retention;
  
 // Begin Renaming Loop
 for( j = NextID; j > 0; j-- )
 {
  var tempSrc;
  var tempDst;
  if( j == 1 )
  {
   tempSrc = srcfile;   
   tempDst = GetFileName( srcfile, 1 );
  }
  else
  {
   tempSrc = GetFileName( srcfile, j - 1 );
   tempDst = GetFileName( srcfile, j );
  }
  fso.CopyFile( tempSrc, tempDst, true );
 }
}

// Returns new file name for archive
function GetFileName( srcfile, id )
{
 var fname = String( srcfile );
 return( fname.replace( /(\.)/gi, id + "." ) ); 
}

// Create a dummy file for testing purposes
function MakeFile( fname )
{
 var f = fso.OpenTextFile( fname, 2, true );
 var msg = "This is a test file: " + new Date().toTimeString();
 f.WriteLine( msg );
 f.Close();
}

// Print Wrapper
function print( msg )
{
 WScript.Echo( msg );
}