Currently there’s no support for multiple applications on the same device, but you can run a multicontainer set of services on a device, akin to docker-compose (and in fact applications are developed using docker-compose manifests). See here for more details on this. Most users find this is more than enough to carry out what they need.
There’s no need to create a new application every time you want to redeploy to a device, in fact the entire balena methodology is that you simply push updates to an application and the device will retrieve these updates and start running them. Any code can be pushed to an application, so even if you completely rewrite your code, pushing to the application will create a new release that is downloaded by the device.
Hope this helps!