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 starting a new stream, you'll get to a step like this:
In the Threads section, you'll specify the number of threads or users you'd like to execute. Note, however, that this will be the number of threads you want per node. The nodes don't know about each other as we're simply starting multiple instances of JMeter on separate machines in the cloud.
On the final step, you'll come across this screen:
where you'll be asked to fill in how many nodes you'd like to start. To figure out how many total users you'll have, multiply the number you put into the Threads field by the number of nodes you start.
For example, if you run 1000 users and then start 10 nodes, you'll end up with a total of 10,000 users. If you want to run a total of 1000 users across 10 nodes, you'd need to set a Threads value of 100. This will run 100 threads per node.