drop down list display

Question

 Hi,

i want a drop down menu to choose how many of that ITEMS they want to purchase. The max number in that box should be the max number of items we have in stock for that base and add. drop down list consist , ,1,2,3,4,5, that way if someone chooses that item and then choose the blank space and it will remove that item from what they want to purchase. For example, if we have 4 items in stock, the drop down box will go blank, 1, 2, 3, 4.

 

Here i need how to write  code for dropdown list to update automatically when stock is reduces.