Django set range for integer model field as constraint

You will have to create a custom validator

from django.core.exceptions import ValidationError

def validate_number(value):
    if something :  # Your conditions here
        raise ValidationError('%s some error message' % value)

And then use this validator in your model

from django.db import models

class MyModel(models.Model):
    field = models.IntegerField(validators=[validate_number])

You can use Django's built-in validators -

from django.db import models
from django.core.validators import MaxValueValidator, MinValueValidator

class MyModel(models.Model):
    qty = models.IntegerField(
        default=1,
        validators=[MaxValueValidator(100), MinValueValidator(1)]
     )

NOTE:

1. The validators will not run automatically when you save a model, but if you are using a ModelForm, it will run your validators on the fields that are included in the form. Check this link for more info.
2. To enforce constraints on a database level refer to maertejin's answer below.

Since Django 2.2 you can enforce the constraints on a database level with CheckConstraint:

class MyModel(models.Model):
    qty = models.IntegerField()

    class Meta:
        constraints = [
            models.CheckConstraint(
                check=models.Q(qty__gte=1) & models.Q(qty__lt=10),
                name="A qty value is valid between 1 and 10",
            )
        ]