The study of motor-related cortical activity is crucial for analyzing brain behavior during motor execution (ME) and imagery (MI). Improving motor learning and recovery in patients with motor disorders involves both ME and MI. Although ME and MI share the same motor brain network, multiple studies show differences in motor-related cortical activity regarding amplitude, timing, and fatigue. Movement-related cortical potentials (MRCPs) and event-related desynchronization (ERD) are key motor-related cortical activities in time and frequency domains. These are used to characterize and monitor neuromotor pathologies through averaging techniques. However, a sufficient number of trials is needed for cortical activity averaging, which may prolong tasks and induce patient fatigue, potentially affecting the results. The aim of this work was to analyze the effect of the number of trials on MRCPs and ERD mu and beta bands during upper-limb movements: elbow flexion/extension, forearm pronation/supination, and hand open/close. Differences between ME and MI were assessed using Monte Carlo analysis of motor-related cortical features, scalp topography activity, and low-resolution electromagnetic tomography (LORETA). The impact of reduced trials varied by movement and feature. Certain differences between ME and MI became statistically nonsignificant with fewer trials. Hand opening/closing and ERD in the mu band were most sensitive to reduced trials. Results were supported by topographic maps and LORETA images, linking reduced trials to increased intersubject variability. These findings highlight the need for an optimal number of trials to ensure reliable outcomes.