Quite simply really. They strangle the resources available to the instances available, based on the number of people they expect to utilise the instances. So, if they expected 1,000 people to use the instance and they only have 10 people to test, the reduce the resources available by a factor of 100, if the instance words poorly with only 10 people then they know they need more resources, if it works well, they know that it should hold up to 1,000 players.
Computers are pretty simple like that. Or they can do what they have done with XI and open a test server, a sneak peek at what is coming and measurable, mass testing of future content.
