A case study design was used to explore a foundational number-sense (FoNS) proficient student’s strategy use in counting. The student, referred to as Petra, participated in a task-based semi-structured systematic counting interview. The interview was analysed qualitatively exploring part-whole reasoning in counting strategies in interrelation with FoNS, which was measured using a digital assessment. The interrelations were discussed as reflecting strands that Kilpatrick et al. (2001) found important for mathematical proficiency. Petra showed the ability to use combinations of one-to-one and relational strategies and high levels of adaptive reasoning and productive disposition. Petra also showed that a number-sense proficient student may be flexible in counting in the 0-10 range but not necessarily in the 10-20 range. Results are discussed as part-whole reasoning on the mental number line.