Jenkins X creates a Preview Environment per Pull Request yeah; which can be as much or as little as you want it to be. e.g. it could be just 1 pod only or could be a suit of related apps (you may want to test multiple things together).

You can define what a Preview Environment is in the source code of your application - its just a different Helm chart really. You can of course opt out of Preview Environments completely if you wish. http://jenkins-x.io/about/features/#preview-environments

Though I've personally found them to be super useful - especially if you are working on web consoles - it lets you try out changes visually as part of the Pull Request review process before you merge to master.