The Law of Diminishing Engineers
Adding an engineer to a team of N creates N new communication channels. Going from 5→6 engineers adds 5 new channels. The overhead: 15-30 minutes per channel per week. At 10 engineers, you lose ~2 full engineering days per week to coordination.
Optimal team sizes: 3-5 for startups, 5-7 for product teams, 8-10 maximum for any single team. Above 10, split into two teams. The coordination cost exceeds the additional capacity.