When planning to deliver a system based on commercial software it is common to start by using manufacturer’s data on the capability of the product to do initial sizing. (e.g. This will support 200 parallel connected users per CPU.) This data often comes from benchmarking and is valid as long as the test conditions are realistically comparable to your production environment.
Where there are no manufacturer guidelines, or as the next stage, you would usually to produce your own early tests. The same issues apply though, unless you are able to work with production-like data on a production-like platform. I am not saying not to use this approach – in fact I strongly recommend doing so – only that care needs to be taken in understanding where the test is realistic. As with other forms of planning, consider the risk carefully and plan in a level of contingency that is appropriate for capacity plans.
If you need help in interpreting manufacturers figures, or in setting up a benchmarking exercise then feel free to contact me at firstname.lastname@example.org, or call on +44 7887 536083.