models.py

class Institution(Base):
    name = models.CharField(
        "Nome da Instituição",
        max_length=100
    )
    city = models.ForeignKey(
        "City",
        verbose_name="Cidade",
        on_delete=models.CASCADE,
        related_name="institution_city"
    )
    tipo = models.CharField(
        "Tipo de Instituição",
        max_length=1,
        choices=constants.INSTITUTION_TYPES,
        default=constants.COURSE
    )
    email = models.EmailField(
        "Contato/Email",
        unique=True,
        help_text="ex: [email protected]"
    )
    telephone = models.CharField(
        "Whatszapp",
        max_length=16,
        help_text="ex: (99) 99999-9999"
    )
    date_visit = models.DateField(
        "Data da Visita"
    )
    period = models.CharField(
        "Período",
        max_length=1,
        choices=constants.PERIODS,
        default=constants.MORNING
    )
    quantity_persons = models.IntegerField(
        "Quantidade de Pessoas",        
    )
    status_visit = models.CharField(
        'Status da Visita',
        max_length=1,
        choices=constants.STATUS_VISIT,
        default=constants.PENDING
    )

    def __str__(self):
        return self.name

    class Meta:
        verbose_name = "Instituição"
        verbose_name_plural = "Instituições"
        constraints = [
            models.CheckConstraint(
                check=models.Q(quantity_persons__gte=1) & models.Q(quantity_persons__lte=20),
                name="quantity_persons_range"
            )
        ]

Testes

def test_quantity_persons_gte_twenty(city):
with pytest.raises(ValidationError):
institution = Institution(
name="Test Institution",
city=city,
tipo=constants.COURSE,
email="[email protected]",
telephone="(99) 99999-9999",
period=constants.MORNING,
quantity_persons=21,
status_visit=constants.PENDING
)
institution.full_clean()