There is some evidence for one more shard on Roshar:
Three of sixteen ruled, but now the Broken One reigns. ”
—Collected on Chachanan 1173, 84 seconds pre-death, by the Silent Gatherers. Subject was a cutpurse with the wasting sickness, of partial Iriali descent.
Unless the three here includes Odium, this implies that there was one more Shard.