In the Hopkins' paper (HOPKINS, Will G. Measures of reliability in sports medicine and science. Sports medicine, v. 30, n. 1, p. 1-15, 2000.) you can find a calculation of sample size based on the number of trials, to keep the same 95% of confidence interval.....please find attached the table from the aforementioned article. In few words, you can use less participants if you have more trials per person, obtaining similar confidence in the typical error....