At Flood, we tend to use the terms "threads" and "users" pretty interchangeably. That's because on most load test tools, one thread simulates one user going through the actions you've set in your script.
When launching your test, you'll specify the number of users you'd like to execute. Note, however, that this will be the number of threads you want per region. The nodes in your flood don't know about each other whether they're in the same region or not, since we're simply starting multiple instances of your load testing tool on separate machines in the cloud.
The total number of users that you run will also depend on the number of regions that you select.
For example, in the screenshot above, two regions have been selected and the number of users per region is set to 1. This works out to a total of 2 users being executed for this flood, which is spelled out in the Summary section.