Architecture - Web Server(apache) and App Server (Oracle 9iAS)

I have a query regarding the web server and application server architecture.
We are desinging the system where there will be many users accessing it. So should we use more than 1 web server(apache) and application server(oracle9iAS) ?
Is clustering needed at both web server and application server level ?
Also let me know ur views regarding the session management in case of multiple web server and application servers.
I mean, if same user is making more than one request and we are using session (using HttpSession or stateful session bean) then how we make sure that all request coming from a same client is going to same web server and application server (assuming we have more than 1 web server and app server)
Thanks in advance.