Use a form to clean the instance

src/wagtail_personalisation/forms.py

@@ -0,0 +1,18 @@
+from django.core.exceptions import ValidationError
+from wagtail.wagtailadmin.forms import WagtailAdminModelForm
+
+
+class SegmentAdminForm(WagtailAdminModelForm):
+    def clean(self):
+        cleaned_data = super(SegmentAdminForm, self).clean()
+
+        rules = [form for formset in self.formsets.values() for form in formset if form not in formset.deleted_forms]
+        consistent = rules and all(rule.instance.static for rule in rules)
+
+        from .models import Segment
+        if cleaned_data.get('type') == Segment.TYPE_STATIC and not cleaned_data.get('count') and not consistent:
+            raise ValidationError({
+                'count': ('Static segments with non-static compatible rules must include a count.'),
+            })
+
+        return cleaned_data