How many projects a Project Manager can manage at the same time?

In this podcast, Ricardo talks about the number of projects that one person can manage at the same time. He says that there is no a specific number, but explains that the most important is the focus that the project manager must have. Ricardo also says that for very large projects does not justify a project manager to share the work with other projects, but in small projects that have some similarity, the project manager could manage multiple projects simultaneously.