Grading of canine mammary carcinomas (CMC) is associated to subjective assessments made by the pathologists. Due to its unbiased nature, stereology can be used to objectively quantify morphological parameters associated with grading and malignancy. However, the use of stereology in CMC has not been fully disclosed. The nuclear numerical density [NV (nuclei, tumor)] is a cellularity‐associated parameter that can be estimated by the optical disector. Herein, it was estimated in 44 CMC and its association with clinicopathologic factors – such as tumor size, histological subtype and grade, vascular/lymph node invasion, nuclear pleomorphism, and survival – was evaluated. Considering all the cases, the mean NV (nuclei, tumor) was 1.6 × 106 ± 0.5 × 106 nuclei/mm3. Lower values were attained in complex carcinomas, comparing to simple carcinomas, in tumors smaller than 5 cm, with low mitotic activity and in those with high nuclear pleomorphism. No statistically significant association with grade or vascular/lymph node invasion was observed, but tumors with disease progression had lower nuclear densities. The NV (nuclei, tumor) and the correlated parameters mirror to some extension those in human breast cancer, suggesting an interesting interspecies agreement. This first estimation of the nuclear numerical density in CMC highlights the feasibility of the optical disector and their utility for objective morphological assessments in CMC. The association between nuclear numerical density and disease progression warrants future studies.