Simply not true. I recently had to salvage business rules from a codebase written by single person over the course of many years. It was probably one of the messiest code I've ever seen.

>"How big is your codebase and how many people work on it"

Depends on a project. On some I work alone. Some had 2-3 persons. The biggest team I've ever had to lead was about 35 people (not all developers). Properly organizing and splitting work and using mostly experienced people the resulting code was very decent and quite possible to grasp. Also well documented.