WebStore was built to meet my needs and then I shared it with others but not much improved or changed over time and is no longer actively developed.
You could edit AdminOrderDetail.aspx and OrderDetail.aspx.
In the repeater where you see
<%# Eval("Name") %>
you could also add
<%# Eval("Quantity") %>
before or after that along with any label or formatting you want.
However that quantity comes from the ws_OrderOffer table and is the quantity of that specific offer in the order, so it won't in all cases correspond to the number of a specific product in an order. Offers may contain 1 or more of a specific product but it would still be 1 offer. So an offer that includes 5 t-shirts won't say 5, it will say 1 because that available quantity field is about how many of that offer are in the order.
If your offers generally have 1 qty of 1 product per offer then displaying that available Quantity field might do what you want. But it won't work correctly in every scenario which is why it was not included.