[Fixed]-Django: limiting model instance based on foregin key

If you want to do some custom validation, you’d better do it via forms, or (as a last resort), using the Model.clean* methods family.

class Campaign(models.Model):
    def clean(self):
        if self.pk:
            if self.item_set.count() > 5 # Or whatever number you need
                raise ValidationError(_('Too much items for me.'))

Override the get_extra() method for your inline:

class ItemInline(admin.TabularInline):  
    extra = 4
    def get_extra(self, request, obj=None, **kwargs):           
        if obj:
            return obj.rows * obj.columns
        return self.extra