Users of the LoadTest features in the eValid suite of tools may already be familiar with the fact that eValid Loadtest runs can involve a number of full-reality Emulated Users (EUs) but also can incorporate HTTP-based activity generators that work like Virtual Users (VUs).
When you want ultra-realistic emulation of actual user activity then you have to use EUs, but in parallel with that many LoadTest runs also involve using a number of VUs -- using an included eValid utility called eVlite.
We have updated the eVlite Limited Fidelity Playback Option Performance Estimates based on recent experiments.
That page includes a table that shows how many VUs an instance of eVlite can generate, and the total number of VUs that appear to be possible on a realtively modest machine.
The numbers show that you can have, on one machine, upwards of 30,000 VUs if you REALLY want to impose a heavy work load on your server. In practice, experienced load test and performance engineers generally choose to have a mix betwen EUs and VUs -- a ratio of about 100:1 seems to be a good mix -- to produce load testing scenarios that are as realistic as possible.