Postgre and Web Request

Hi,

I am a new user of PostgreSQL and there are some questions about its
performance in a scenario with a high requisition rate

Lets picture an imaginary scenario:
In my system (Debian Linux), there are 200.000.000 records on the
database, and a total number of 10.000 diferent users.
(the manual stated the following: there is a main process
called postmaster. It starts new processes to each different request and
each different user ... I dont understand very well ... please correct me
if I`m wrong)
If all users try to access, through WEB, at same time the database,
what happens:
1. With the OS? Will it crash?
2. Will the Postmaster process startup 10.000 diferent processes
at the same time?
3. What about performance? Is there any peformance downgrade?
4. What is the best solution for this problem?
5. How many simultaneos requests may the Postmaster open
withouth decreasing perfomance?

Grettings,
Kilmer Cruz