[Answered ]-Count common ManyToMany members in queryset

You can count with:

from django.db.models import Count, Q

Foo.objects.annotate(
    common_count=Count(
        'bars',
        filter=Q(bars__foo=main_object)
    )
)

or if you only want to retrieve objects that have at least one Bar in common:

Foo.objects.filter(
    bars__foo=main_object
).distinct()