When your setting up an production environment you will have to think about some additional issues such as can only one server handle all the traffic also what if that server goes down, the production environment should be highly avaialble that means there should not be any single point of failure.
The WebSphere Application Server has a concept of cluster in which you have identical copies of servers and you combine them in one cluster. Take a look at this sample topology in this case we have three portal server as part of one cluster. The Web Server is responsible for distributing the user requests among the server and Deployment manager is responsible for administration of different servers in the cluster.
There are multiple advantages of doing that
1) You can handle large load, in this case you will install HTTP server infront of your application servers and this Http Server can play a role of load balancer by distributing the load between different servers in the cluster. The HTTP server also makes sure that if the request from client A was handled by server 1 then the next request from client A should go to the same Server 1
2) High availability: If one of your application server goes down then the Http server will figure that out and will stop forwarding requests to that server. The session data saved on server1 is also made available on other server so that they can continue the transaction
3) Single point of administration: The Deployment Manager will allow you to install or update application which will be available on all the server in that cluster. You can administer resources through either WAS Admin Console or through scripting
Since WebSphere POrtal is nothing but an enterprise application deployed in WebSphere Application server all the clustering concepts of WebSphere Application Server also apply to the WebSPhere POrtal server.