I do net get your code totally but I think you forget to give an initial value to $i and afterwards you forget to add 1 to $i in every loop you make
something like:
 Code:
$folder = "U:\test\"
$Dir = dirplus($folder,"/a-d /f tif")
$i = 1

$date = Trim(Join(Split(@DATE,'/'),'-'))

For Each $file In $Dir
		 $file.Move($folder + "AND" + $date + "-" + $i + ".tif")
		? @SERROR
		$i = $i + 1
	
Next