<p>Cultivating and enhancing the research competence of doctoral students is a crucial aspect in improving the quality of doctoral education. Academia has acknowledged the role of interpersonal interactions and team composition of fostering doctoral students’ research competence. However, ongoing debates persist regarding whether the impact of team size on doctoral students is characterized by a supportive or congestion effect. Based on the data obtained from the “Survey of PhD candidates in 2020 in China” (<i>N</i> = 709), this study focused on the protégé size composed of co-mentees who share supervisors and found that the correlation between protégé size and research competence follows a quadratic inverted U-shaped pattern. It indicates that during interactions with co-mentees, both mutual progress and congestion coexist, but there is an optimal protégé size (OPS) for enhancing the competence of doctoral students. When the protégé size is less than OPS, co-mentees have a supportive effect as the size grows, while when the protégé size exceeds OPS, co-mentees exhibit a congestion effect. In China, OPS is ten, but it differs with discipline, gender, and initial competence. Furthermore, the impact of protégé size is mediated by the supervision frequency and quality provided by supervisors. Against the backdrop of the continuous expansion of doctoral enrollment in China, this research underscores the importance of maintaining a stable size and fostering doctoral students’ research competence from the perspectives of institutional policy and supervision, which extends the literature on doctoral practices and will stimulate debate about co-mentee learning as a strategy for supporting doctoral students.&#xa0;</p>