I think you have to modify the CheckboxCombo_ComboHandler() function located in the CheckboxCombo.au3 file like this:
1) add a variable ($iTotChecked) in line 315:
Local $sText = "", $j = 0, $iTotChecked = 0
2) change line 323 from this
If $sText Then $sText = StringLeft( $sText, StringLen( $sText ) - 2 )
to this
If $sText Then
$sText = StringReplace(StringTrimRight($sText, 2), ',', ',')
$iTotChecked = @extended + 1
EndIf
3) change line 325 from this
DllCall( "user32.dll", "int", "DrawTextW", "handle", $hDC, "wstr", $sText, "int", -1, "struct*", DllStructGetPtr( $tDIS, 8 ), "uint", $DT_SINGLELINE+$DT_VCENTER+$DT_END_ELLIPSIS ) ; _WinAPI_DrawText
to this
DllCall( "user32.dll", "int", "DrawTextW", "handle", $hDC, "wstr", String($iTotChecked), "int", -1, "struct*", DllStructGetPtr( $tDIS, 8 ), "uint", $DT_SINGLELINE+$DT_VCENTER+$DT_END_ELLIPSIS ) ; _WinAPI_DrawText