Django: Limit number of model instances to be created

gsevl.py

def validate_max_rows(obj):
    max_num = 5
    model = obj.__class__
    if (model.objects.count() == max_num and obj.id is None):
        raise ValidationError(u"oops! max num is %d" % max_num)
        
class Model(models.Model):
    # Fields
    def clean(self):
        validate_max_rows(self)