I believe the answer to that is largely going to depend on the software that is running your portal and your user requirements. Some applications are written very efficiently and don't require a huge amount of resources to run (memory, CPU, disk, bandwidth, etc.). In a multi-tier setup (web server, application server, and database server), the size of your server is generally going to get larger the further down you go (application server will be bigger than the web server, database server will be bigger than the application server).
Another factor to consider is the number of concurrent users. If you are expecting a lot of those 10,000 users to be on the system at the same time, you can probably expect that you will need a pretty large web server. Realistically, you should probably be looking at load balancing which will help in both performance and scalability.
So really, the short answer to your question is...it depends. You will need to make sure you have clear requirements related to performance and up-time before deciding on the exact server infrastructure that will run your application. If it is an off the shelf application then the vendor documentation should be able to provide some guidelines on how best to scale each tier.