The first thing you need to work out is what load the solution needs to be able to support. There are a number of elements to this including standard load, peak load, and expected traffic. If you are replacing an existing solution, the best place to start is the traffic of the existing solution. In particular, what you want to look for are the peak loads. For example, the F1 website that we worked on had a particular pattern for traffic. For 11 months of the year, there was a medium load, during the two weeks leading up to the event and during the event, the peak increased by a factor of 20. So, any testing done had to work for the peak load. If that was covered, then the rest of the year, the performance of the solution would be more than enough to cope with the traffic.
The specific factors that need to be considered are set out below.