This 22-session constructivist teaching experiment set out to investigate a preschool student's number understanding relative to his subitizing activity. Subitizing, a quick apprehension of the numerosity of a small set of items, has been found to characterize perceptual and conceptual processes students rely on as their understanding of number develops. The purpose for this study is to investigate how a preschool student's, Ben, perceptual subitizing activity changed relative to the density of items and the development of his number understanding. Findings indicated that early on in the teaching experiment, Ben's perceptual subitizing activity was influenced by his primary reliance upon the perceived amount of space between items. Shifts in reasoning when perceptually subitizing indicated physiological and experiential development in Ben's number understanding, as Ben described the number of items increasing when the perceived amount of space between items decreased. Number conservation was considered as relevant to these findings because Ben's explanation for why a number could increase or decrease mirrored similar logic when unable to conserve number. Implications of this study suggest nuances in number understanding development which can explain preschool students' reliance upon a more refined set of perceptual subitizing.