how to print a form in twig where a variable number of form elements have been printed already?
If you look at form.html.twig in the Drupal Core's system module, you'll see:
{#
/**
 * @file
 * Default theme implementation for a 'form' element.
 *
 * Available variables
 * - attributes: A list of HTML attributes for the wrapper element.
 * - children: The child elements of the form.
 *
 * @see template_preprocess_form()
 *
 * @ingroup themeable
 */
#}
&lt;form{{ attributes }}&gt;
  {{ children }}
&lt;/form&gt;

Using this as a guide, you should be able to do:
&lt;form{{ attributes }}&gt;
  {% for item in form|children %}
    {{ item }}
  {% endfor %}
&lt;/form&gt;

children is already a variable in the template so you can just use it like:
&lt;form{{ attributes }}&gt;
  {% for item in children %}
    {{ item }}
  {% endfor %}
&lt;/form&gt;

But if you're not wrapping the individual children, why even use a custom template? Why not just use the default form.html.twig.