If X and Y are two elements which belong to the same group of Newland's periodic table, how many elements exist between X and Y?

The law of octaves states that every eighth element has similar properties when the elements are arranged in the increasing order of their atomic masses. This means X and Y have identical properties since they are in the same group.
 Newland’s table will look like X, 1, 2, 3, 4, 5, 6, Y.

Hence, 6 elements exist between X and Y.