Oh, I think I totally read your post wrong.
If you don't mind another page load then unsurprisingly you need two forms. You can use get variables to have one PHP file with two 'modes' of operation, or you can have two files, one for each job.
So say first.php without any get data outputs a drop down numFields and a next button, submits the result back to first.php.
Then first.php with the get data does a loop with $ii=intval($_GET['numFields']); (for instance), outputs a field for each iteration, and submits to second.php to do whatever else.
If I ditch the drop-down idea and instead just have them input and then submit a number between 1-10 is that then do-able in PHP using a loop?