It is very unlikely that you are going to need to scale Redmine very much. Needing to scale it would mean that you have hundreds or thousands of users working simultaneously on the system. However, after using Redmine intensively for several years, you might end up scaling it to improve response time.
Rule out the possibility that something else (such as network problems, virus, faulty hardware, or misconfiguration) is causing your server to work slowly or misbehave.
As you obviously made sure that you need to scale up Redmine, here is one technique on how scaling out (preferably on physical servers) can be achieved.
If you are facing high server loads, make sure to check your ...