Query for top x elements in Django

I have two models such that

class JobTitle(models.Model):
     name = models.CharField(max_length=1000)

class Employer(models.Model): 
     jobtitle = models.ForeignKey(JobTitle,unique=False,null=True)

As you see, one employer may have many jobtitles. I try to make a query to get top 5 employers whose number of job titles is maximum

How can I achive this is Django ?

Thanks