Kind of the wrong question. It's more about qualitative sample selection and the expected effect size than it is the number of subjects. Based on my admittedly

Kind of the wrong question. It's more about qualitative sample selection and the expected effect size than it is the number of subjects. Based on my admittedly poor understanding, I would expect to see a sample size of around 70 to have the power to detect a small effect in a single-tailed T test. I would also expect to have the samples selected and balanced carefully, preferably from the target group (i.e. experienced professional programmers in the language at hand) From what I can tell, for an effect to be detected at least 80% of the time in a sample of 20 people, you'd need the effect to be >60%. I would definitely not expect that a short term project would show 60% improvement when employing similar levels of testing, changing only either before or after writing the code under test.