The goal of the current study was to examine the role number of children in a family has on the discrepancy between mother- and teacher-reported externalizing problems. A total of 243 youth and mothers (Mage = 9.92 ± 2.84 years; age range: 8–16 years; 129 males and 114 females) presented for a psychological evaluation. Behavioral reports were gathered from the Child-Behavior Checklist and the Teacher Rating Form. A factorial analysis of variance first tested for mean-level discrepancies between informant reports across four sibling groups. Next, a polynomial regression analysis tested rank-order discrepancies as a function of sibling groups. Results indicated that mother-teacher mean-level discrepancies did not differ as a function of the number of children in the household. In contrast, rank-order discrepancies did differ across sibling groups; mothers with fewer children had higher congruence with teacher-reported externalizing problems. Overall, mother-teacher rank-order agreement was greatest when the child was an only child and decreased modestly with the first and each additional sibling. We discuss these findings in light of previous literature and provide direction for future literature examining mother-teacher informant discrepancies.