Numbers are an integral part of our daily lives, essential for making sound decisions. Surprisingly, numerical abilities, often termed number sense, begin developing early in life, shaping our foundational understanding of mathematics. This chapter explores the concept of number sense, demonstrating that even young children exhibit sensitivity to numerical magnitudes in everyday problem-solving scenarios. We delve into the transition from non-symbolic to symbolic numbers and its impact on brain development. As children acquire symbolic numerical skills, brain regions supporting number sense are influenced, and experiences refine these representations. We also explore individual differences in mathematical competence and their neural correlates. Furthermore, we discuss the implications of math interventions on brain development, emphasizing the importance of nurturing numeracy skills from an early age. This understanding has far-reaching implications for education policies, ensuring that every child has the opportunity to unlock their numerical wisdom. This chapter illuminates the journey from number sense to mathematical mastery in the developing mind.