3 people so far is my largest team in a professional setting, however we work with a large and diverse codebase that's been in development for about 10yrs(at least that's how far back our commit log goes).
The repository consists of 750 C++ source files, 400 C++ header files, 100 ruby scripts, 5 java source files and countless shell scripts that I can't be bothered to count. A lot of the C++ source files are unit test files, thus no header to match.