The article explores the impact of multi-tenant and single-tenant designs on the scalability of software architecture, emphasizing the benefits and drawbacks of each approach. Single-tenant architecture, as exemplified by FusionAuth, offers predictable performance under load, enhanced security through data isolation, and a straightforward horizontal scaling process via its stateless design, making it suitable for handling high-volume applications and ensuring compliance with data residency requirements. In contrast, multi-tenant systems allow for serving multiple customers on shared infrastructure, which is cost-effective but poses security risks and performance challenges due to potential load spikes from "noisy neighbors." The text also highlights the importance of considering scaling strategies, such as vertical and horizontal scaling, to accommodate growing user bases and address authentication bottlenecks caused by CPU-intensive processes like password hashing. Various real-world examples, including the failures of Pokémon GO and ChatGPT to scale effectively, underscore the necessity of preparing for sudden traffic spikes and maintaining infrastructure that can handle peak loads to prevent service degradation and protect user data.