When you install FileMaker Server, you have the option of using one server, or two. If you're using two, one machine is the database server, and one runs the Web Publishing engine and the web server. The database server is the master, and the other is the worker.
With regard to WebDirect, 5 is not the limit, 100 is the maximum. You indicated that 5 will work for you, so you'd need to purchase one set of concurrent connections, which are sold in groups of 5.
Two good links to look at. The first discusses deployments such as what you're describing, and the second provides specs and recommendations. In the second, you'll see a table showing recommendations. It shows that if you anticipate 1-6 WebDirect simultaneous users, a second machine for web publishing is not recommended.