It needs to be a decision problem (or easily recast as a decision problem). "given a number as input, output as many 1's as that number" doesn't have a yes or no answer. You could try to ask a related question like "given a number as input and a list of 1s, are there as many 1s as the number?" but that has a very large input.

Straightforward to pose as a decision problem -- you're given a sequence of {0,1} representing a base-2 input (N), output a string of {0, 1} such that there are N 1's. You could make it "N 1s followed by N 0s" to avoid being too trivial, but even in the original formulation there are plenty of "wrong" answers for each value of N, and determining whether a witness is correct requires O(2^b) time, where b is the number of bits in N.