As an aside, I wouldn't offer 13 of anything. Why appeal to people's biases?
That aside, I don't know if there's a magic number of not. But, from a marketing standpoint, keep in mind that there's no reason you have to offer your complete line at one time. That is, say you develop several more cookies, for a total of 20. You could have a base of, say, 15 that get offered every day, with one or two of the others being rotated as daily specials.
Not only does this make things more managable for you, from a production standpoint, it encourages daily visitation by people anxious to try that day's special.