There are two closely connected laws in chemistry, the law of definite proportions, or Proust's law and the law of constant composition. The former sates that all chemicals contain the same proportion of elements by mass and the latter states that any sample of a given chemical compound will always have the same elemental composition.
In the case of KCl, the elemental composition is one atom of potassium (K) and one atom of chlorine (Cl). This is maintained for all samples of KCl.
Therefore the completion of the statement you have provided, any two samples of KCl will have _____, according to the law of constant composition is, "the same elemental composition."