In Enterprise IT the thing you buy probably already has a ‘load profile’ and recommendation regarding concurrency, however the integration and callouts will not get a real test until you put the thing live on site.

For a system that is large, distributed, or highly interactive even small numbers of users should be part of your planned load testing

Perhaps the following abbreviations might save you some typing when planning

3USR – 3 users (real)
3USS – 3 users (simulated)

5USR – 5 users (real)
5USS – 5 users (simulated)

25USR – 25 users (real)
25USS – 25 users (simulated)

…and so on…