class CustomListSerializer(serializers.ListSerializer):
    def __init__(self, *args, **kwargs):
        count = kwargs.pop('count', None)
        self.count = count
        super().__init__(*args, **kwargs)

    def to_representation(self, data):
        if self.count is not None:
            if len(data) != self.count:
                raise serializers.ValidationError("Incorrect list size")