[Answered ]-Require at least one MultiFieldPanel object [wagtail]

InlinePanel takes an optional min_num argument.

Finally, min_num and max_num allow you to set the minimum/maximum number of forms that the user must submit

From http://docs.wagtail.io/en/v1.10.1/reference/pages/panels.html?highlight=min#inline-panels-and-model-clusters