Comment 4 for bug 1549517

// char * ngettext (const char * msgid, const char * msgid_plural,
         unsigned long int n);

show_notification (_("Task finished"), N_(counter.to_string () + " tasks have been removed", "A task has been removed", counter));

Of course, you can do better and use some format strings, since fmt strs can give translators more flexibility and more sense about what they are translating. See https://www.gnu.org/software/gettext/manual/gettext.html#Preparing-Strings. Unfortunately I don't want to look at things like snprintf for now.