: The reported incidence of multiple breast cancers varies widely, ranging from 6 to 60%, depending on the definitions used and methods of detection. With advancements in preoperative imaging techniques, such as magnetic resonance imaging, the detection of multiple breast cancers has improved. However, the clinical significance of multiple breast cancers remains controversial, with conflicting results regarding their impact on prognosis. We investigated the association between the number of synchronous ipsilateral T1 breast tumors, overall survival (OS), and breast cancer-specific survival (BCSS). : We retrospectively analyzed 45,881 patients diagnosed with invasive breast cancer who underwent surgery between 2004 and 2016. The patients were categorized based on the number of tumors: one ( = 43,234), two ( = 2241), and three or more ( = 406). The OS and BCSS scores were compared across the groups. : There were no significant differences between the one- and two-tumor groups ( = 0.490 and = 0.650, respectively). However, patients with three or more tumors had significantly lower OS and BCSS rates than those with one or two tumors ( < 0.001 for both comparisons). Multivariate analysis confirmed that the number of tumors (three or more) was an independent risk factor for poor OS and BCSS. : Our findings suggest that patients with synchronous ipsilateral T1 breast cancers and three or more tumors may benefit from escalated treatment strategies due to their potentially worse prognosis.