The rapid growth of the Internet of Things (IoT) demands efficient system architectures and protocols to ensure consistent performance at scale. This paper explores the scalability of IoT systems across three key layers: sensing, network, and control. IoT scalability is the ability of a system to maintain consistent and reliable performance despite a continuous increase in connected devices. To evaluate scalability, we introduce the scalability indicator (SI), a metric designed to assess an IoT system’s scalability capability. Through extensive research and real-world deployments, we identify key challenges in data sensing, routing, and system control. Our study presents a model to understand these challenges and proposes strategies to optimize resource utilization, ensuring efficient data collection. The findings also emphasize the key influencing factors for the stable performance of large-scale IoT systems, providing valuable insights for how to design scalable systems that can meet the growing demand for interconnected devices.