users = User.objects.all()
    for u in user:
        print(u.name, len(u.comments))

This is fine if you are working with a small data set. It is inefficient, but if it's quick enough, readability trumps efficiency IMHO.

Django ORM has a succinct way of doing the "SELECT COUNT(*)" pattern:

    users = User.objects.all()
    for u in user:
        print(u.name, u.comments.count())

And you can use query annotations to get rid of the N+1 query issue altogether:

    users = User.objects.annotate(n_comments=Count("comments"))
    for u in user:
        print(u.name, u.n_comments)