Adding controls at runtime

[Mr. Bean]

Hello,

I'm making a small web application that allows the user to select a
submit for one or more items. Is there a way to allow the user to add
controls at runtime in the following manner:

--------------------------------------------------------------------
item | avialable number | quantity
--------------------------------------------------------------------

the user selects an item(dropdown list) the available number field is
filled by a small funtion trigerred by the item_listbox change event. I
was able to add a fixed number of lines but i received complaints that
users might have variable number of lines per request.

Thanks