For any light, say, light k, it only gets switched when the interval is a factor of k. Now, notice that factors come in pairs, that is, if a*b==k, you will switch light k at both intervals a and b. As long as a !=b, the light k will be off in the end. The only chance that light k remains on is when a==b, that is, k==a^2, , in this case it only gets switched once for this pair of factors. So, k needs to be a square number!
Then the question is really asking how many square numbers are smaller or equal to N, which is trivial to programmers.