Deis allows you to run apps in two different ways: using a Procfile, and using a Dockerfile. When using a Procfile, you define what type of processes you'll be running, such as:
web: node index.js
To scale this, say to 2 instances, it's as simple as:
$ deis ps:scale web=2
This doesn't work when using the Dockerfile approach however. For that, we can see from ps:list that the instance type is actually cmd.
$ deis ps:list --- cmd: cmd.1 up (v2)
If we were using a Procfile, this would say web.1 up (v2) etc. So to scale the Dockerfile, we need to use cmd rather than web.
$ deis ps:scale cmd=2
And then, once it's got coffee, it'll scale your apps.