Couchbase Server decouples RAM from the I/O layer. This is a huge advantage. It allows you to scale high at very low and consistent latencies. It also enables Couchbase Server to handle very high write loads without affecting your application's performance.
However, Couchbase Server still needs to be able to write data to disk. Your disks need to be capable of handling a steady stream of incoming data. It is important to analyze your application's write load and provide enough disk throughput to match.
While information is written to disk, the internal statistics system monitors the outstanding items in the disk write queue. From its display, you can see the disk write queue load. Its peak shows how many items stored in Couchbase Server would be lost in the event of a server failure. It is up to your own internal requirements to decide how much vulnerability you are comfortable with. Then you size the cluster accordingly so that the disk write queue level remains low across the entire cluster. Adding more nodes will provide more disk throughput.
Disk space is also required to persist data. How much disk space you should plan for is dependent on how your data grows. You will also want to store backup data on the system. A good guideline is to plan for at least 130% of the total data you expect. 100% of this is for data backup, and 30% for overhead during file maintenance.