Our company has several dozens of top qualified in-house developers. Usually, we form a team for the customer's project. For small projects, it's 2-3 developers. For big projects, it's 4-5 developers or even more. Very rare we have a project where only one developer is required.
Every project is under the control of one key developer who checks every day's commits, plans the software architecture, and decides what task should be executed by every team member. Because key developers have years of experience, we can avoid many errors and be sure that customers' tasks will be executed successfully.