I think you might have been referring to this: https://en.m.wikipedia.org/wiki/Euler%27s_sum_of_powers_conjecture . As you can see, it was once conjectured to be the case by Leonhard Euler that you would need 2 squares to add up to a square, 3 cubes to add up to a cube, four 4th powers to add up to a 4th power, etc. In 1966, however, a computer search revealed a counterexample: namely, there are four distinct (positive) integers whose fifth powers sum up to a fifth power.