Use existing relationships with customers to recruit users. When contacting the customer, it is very important that you emphasize that it is not a product presentation, that the product is not finished, that it is being developed, and that only people fitting the target profile should take part.
When you invite a group of users for parallel testing, make sure that you avoid hierarchical problems within a company and competitive situations between companies.
There should be between three and six test users. This is true for single and parallel testing. In single testing, it is a common experience that the amount of new information discovered decreases as the number of users increases. If there is iterative design done between consecutive User Days, the number of testing sessions should be decided ad hoc depending on the results.
For parallel testing, the number of participants depends on the number of sessions that can be performed in parallel on the User Day. The group discussion in the final Usability Round Table will be more successful if there are more people participating. If you can only organize three parallel test sessions, you should consider having six participants test in groups of two on one machine.
If there are fewer machines available than there are participants, you can also include other activities to occupy the rest of the participants. For example, you could include an interview about working practices or present the current results of your user/task analysis.
Source: User Day Toolkit