models.py

class Occupation(models.Model):
    event = models.ForeignKey(Event, related_name='sections')
    section = models.ForeignKey(Section)
    expected_capacity = models.PositiveIntegerField(
        help_text=)

    def __unicode__(self):
        return self.section.name

class OccupationForm(forms.ModelForm):
    class Meta:
        model = Occupation

    def __init__(self, *args, **kwargs):
        super(OccupationForm, self).__init__(*args, **kwargs)
        self.fields['expected_capacity '].help_text = 'Max Capacity: %d' % self.instance.section.crowd_capacity