Dunbar's number
Dunbar's number, proposed by anthropologist Robin Dunbar in the 1990s, is a cognitive limit on the number of stable social relationships a human can maintain, approximately 150 for casual acquaintances, with tighter inner circles at 50, 15, and 5.
The applied implication: at 5-15 engineers, everyone knows everyone's project and decisions can be made in the kitchen. At 50, a single weekly all-hands keeps coherence. At 150, you need explicit org structure, documented decisions, and process. The informal coordination that worked at 50 collapses, often messily. Companies that scale from 50 to 200 engineers in a year report this as the most disruptive transition: technical talent that thrived in the small-team mode struggles in the formal-process mode, and process structures that worked at 50 (e.g., a single shared standup) become absurd at 200. Forewarned teams plan for the transition deliberately; many discover it retrospectively.