The accessory navicular is an extra piece of bone on the medial side of the navicular bone.
This study reviewed over 1000 x-rays and found an accessory navicular in 23.3% of them. Others have reported that the prevalence is more like 10%.
It is not totally clear just how common an accessory navicular is and research on it may classify the presence of absence of the extra bone differently. This will affect how common it is when reporting on it.
The number of accessory navicular bones that are actually painful is quite low and most are never a problem.