Dynamically add value to Drop Down List...

I have a drop down list that is being populated from the database with the values (1, 2, 3, 4, 5).
How do I dynamically add an additonal number based on the last item count? For example DropDownList SortNumber     1     2   <----  This is the Selected value ( but any number can be the selected number)     3     4     5     6   <----- How do I add # 6 to the drop down dynamically and the Select value will be # 2
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