diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index a3d68b03..a734f5fe 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -162,6 +162,18 @@ class BaseSerializer(Field):
             key: value for key, value in kwargs.items()
             if key in LIST_SERIALIZER_KWARGS
         })
+        
+        # Validate instance/data length mismatch early for many=True
+        if 'instance' in kwargs and kwargs['instance'] is not None:
+            instance = kwargs['instance']
+            data = kwargs.get('data', [])
+            if not isinstance(instance, models.manager.BaseManager) and hasattr(instance, '__len__') and hasattr(data, '__len__'):
+                if len(instance) != len(data):
+                    raise AssertionError(
+                        'Mismatched instances and data items (%s instances for %s data items)' % 
+                        (len(instance), len(data))
+                    )
+
         meta = getattr(cls, 'Meta', None)
         list_serializer_class = getattr(meta, 'list_serializer_class', ListSerializer)
         return list_serializer_class(*args, **list_kwargs)
@@ -683,7 +695,18 @@ class ListSerializer(BaseSerializer):
         ret = []
         errors = []
 
-        for item in data:
+        for idx, item in enumerate(data):
+            # Set the instance on the child serializer to the corresponding item
+            # when validating in the context of many=True
+            if self.instance is not None and isinstance(self.instance, (list, tuple)):
+                child_instance = self.instance[idx]
+            else:
+                child_instance = getattr(self, 'instance', None)
+            
+            # Temporarily bind the child instance
+            old_child_instance = self.child.instance
+            self.child.instance = child_instance
+            
             try:
                 validated = self.child.run_validation(item)
             except ValidationError as exc:
@@ -691,6 +714,9 @@ class ListSerializer(BaseSerializer):
             else:
                 ret.append(validated)
                 errors.append({})
+            finally:
+                # Restore the original instance
+                self.child.instance = old_child_instance
 
         if any(errors):
             raise ValidationError(errors)