To complete Marc's answer, going with one central repository (with all the projects inside) does not prevent multiple repositories, each with their own project.
Let's hear it from Linus himself:
There are no real issues either way, and perhaps more importantly, it's
not even something you have to decide on day one.
You can easily do it one way or the other, and either
- switch around as needed
- or even mix the two approaches where it makes sense.
For example, it may be entirely sensible to have the common "distribution
points" use a single shared repository that contains all modules as
separate branches within the same repository.
But even though such a central distribution point repository is set up
that way, individual developers may well decide that they are happier
having separate repositories for different modules. The two approaches are
not mutually incompatible, and you can fetch data and push it back out
between both different kinds of repositories.