//I reckon it's 17: 'seven' has 5 letters, 'eleven' has 6 letters, 'fifteen' has 7 letters, 'thirteen' has 8 letters, and 'seventeen' has 9 letters.// Given that a similar solution as been given in a previous puzzle its probably what the setter wants.
But,,, seventeen is not the only number with 9 letters. It could be 24 (twentyfour) or 25 or 29 or 31 or 32 or....
I think it work's tho if you say its the smallest number that fit's the pattern