[cflsystems]

In the template you can probably do something like
if number_in_text_box equal $product.avail then show message

Not sure about exact variables available there right now. If this works note that if you have 5 items available and cutomer enter 5 or mre then 5 they will see same message so you may wanna change the message a little bit to more generic one
If this doesn't work you have to edit cart.php and find the code that takes the number entered and compares to product availability