Answer: 5

The numbers are 1, 2, 3, 11 and 17. Note that, because 2 is the only even prime, the only way we can sum two primes to give an odd number is to sum the odd number before it (if it is a prime) and 2.

One of the oldest unsolved problems in number theory is Goldbach's conjecture, which says that every even number (except 2) can be written as the sum of two primes. Perhaps you could be the first to prove it...