Comment 5 for bug 1549517

Oops, I shouldn't be looking at the C gettext manpage.

Regardless, there is ngettext in Glib (vala):
http://valadoc.org/#!api=glib-2.0/GLib.ngettext
// public unowned string ngettext (string msgid, string msgid_plural, ulong n)
// basically the same thing

And you should be able to do format strings like this now:
show_notification (_("Task finished"), N_("%d tasks have been removed", "A task has been removed", counter).printf(counter));