The current study investigates the development of the conceptual subitizing skill, the ability to quickly and accurately recognize quantities above 4 items, and its relation to mathematics achievement in Grade 2 students. Both reaction time (RT) and strategy use to enumerate patterned structures of 4 to 10 dots were examined. Using cluster analysis on the collected enumeration speeds, three distinct groups were identified that supports the acquisition of conceptual subitizing as a developmental process. These clusters additionally aligned with students’ self-reported enumeration strategies used during the task, and also predicted children’s mathematics achievement. As one of the first psychophysical investigations into the conceptual subitizing skill, this study provides preliminary evidence for the importance of focusing on children’s speeded enumeration of larger set sizes (e.g., between 4 and 10) as a potential influence on their mathematical abilities.