The architecture of XPM is such that part of it is within your website build (ADF), part is the Session Preview Web Service, part is a dependency on the Content Manager, and, of course, the underlying databases (CM DB, Broker DB and the Session Preview DB).
So if you need to scale it out, you'll need to consider all these parts. Think through why you're scaling out: fail over, high availability performance or all), and scale out what you need (based on content editors'/users' actions). If doing a lot if previewing, then reinforce your Preview Webservice and DB (as per the documentation Eric provided links to). If doing a lot of saving of pages and components, then look the CM servers.