Hi Mark,
It will greatly depend on what those pages in those 100 URLs are doing. If those are simple operations then maybe a single blade will do. Employing caching also helps.
OpenACS does support the load balancing / clustering that you are looking for in adding capacity out of the box. By adding more front end boxes you may run into that the db server will be the bottleneck. So employing caching will be needed when you run into this.
