What is the total number of Shelves in a stack?

Study the following information carefully and answer the below questions. Certain numbers of boxes are placed in a stack one above another. No two boxes are placed in a same shelf. Each boxes has different number of Pens. Four boxes are placed between Q and T which is placed below box R. Not more than four boxes are placed above Q. Three boxes are placed between T and the box which has 24pens which is placed above T. The Box P is placed at topmost position. There are six boxes are placed between P and the box which has 40pens.The number of boxes placed between box Q and the box which has 40pens is half of the number of boxes placed below box T. Box R is placed just above box S. Six Boxes are placed between S and U which is placed three position below the box which has 36pens. Box U is placed second position from bottom. Eight boxes are placed between R and the box which has 12pens.