How many types of namespace are there in c++

Answer 1

In fact, as much as necessary.
But I do not advise you to use more than 2 connected namespace in large projects, because then it will be difficult to debug problems.

Answer 2

A banal example:
suppose, you use only the standard library and boost, so you decided to write everywhere:

using namespace std;
using namespace boost;

...and now there is a new version of the standard library, in which many classes are transferred from boost. And suddenly your code no longer compiles.
Draw conclusions, bro)