The error-related negativity (ERN) and error positivity (Pe) are increasingly being examined as neural correlates of response monitoring. The minimum number of error trials included in grand averages varies across studies; indeed, there has not been a systematic investigation on the number of trials required to obtain a stable ERN and Pe. In the current study, the ERN and Pe were quantified as two random trials were added to participants' (N=53) ERP averages. Adding trials increased the correlation with the grand average ERN and Pe; however, high correlations (rs>.80) were obtained with only 6 trials. Internal reliability of the ERN and Pe reached moderate levels after 6 and 2 trials and the signal-to-noise ratio of the ERN and Pe did not change after 8 and 4 trials, respectively. Combined, these data suggest that the ERN and Pe can be quantified using a minimum of between 6 and 8 error trials.