I’ll mention two. My primary art (Nihon Goshin Aikido) is obscure compared to many others. At its height, there were probably fewer than 80 schools. Not tiny, but pretty obscure compared to Ueshiba’s Aikido, for instance.
And for a more obscure art, I had a chance to attend a seminar in Yanagi-Ryu Aiki Bugei. If the origins I heard are accurate, it was once a family style. I think the Soke (actually the appropriate title for an inherited style, I think) never had more than a few students at a time.