How can I make a stylesheet that counts subcells?

I think when it's done each time you save the notebook it should be nice enough :)

SetOptions[
 EvaluationNotebook[],
 NotebookEventActions -> {
   {"MenuCommand", "Save"} :> (Scan[
      Module[{nr},
        SelectionMove[#, All, CellGroup, AutoScroll -> False];
        nr = Length @ Select[
           SelectedCells[], 
           Experimental`CellStyleNames[#] === "Input" & (*1*)
        ];
        SetOptions[#, CellDingbat -> "(" <> ToString[nr] <> ")"];
      ] &
      ,
      Cells[CellStyle -> "Section"] (*2*)
   ]),
   PassEventsDown -> True
   }
 ]

Ad 1. Cell style to count

Ad 2. Cell style whose parent group end "resets the counter"

You can use it in stylesheets too.

---

Update from Question's Author:

As Kuba's comment fixes the raggedness:

SetOptions[EvaluationNotebook[], 
     NotebookEventActions -> {{"MenuCommand", 
         "Save"} :> (Scan[
          Module[{nr}, 
            SelectionMove[#, All, CellGroup, AutoScroll -> False];

            nr = Length@
              Select[SelectedCells[], 
               Experimental`CellStyleNames[#] === 
                 "ItemNumbered" & (*1*)];

            SetOptions[#, 
             CellDingbat -> 
              Cell[BoxData[
                PaneBox[
                 StyleBox[ToString[nr] <> "   ", 
                  RGBColor[0.5, 0.5, 0.67, 0.81], 
                  FontFamily -> "Continuum Light", 15], 
                 Alignment -> Right, ImageSize -> 40]], 
               Background -> White]];] &, 
          Cells[CellStyle -> "Subsection"] (*2*)]), 
       PassEventsDown -> True}]