The number of elements in the array.
If the SerializedObject contains a single object, this property returns the number of elements in the array.
If the SerializedObject contains multiple objects, this property returns the smallest number of elements. This is done so that iteration of the SerializedObject only exposes properties found in all objects.
If there are multiple objects and the smallest array size is larger than SerializedObject.maxArraySizeForMultiEditing then this property returns 0 and the elements cannot be retrieved with SerializedProperty.GetArrayElementAtIndex.
Additional resources: isArray, minArraySize, GetArrayElementAtIndex.