One of the big advantages that Couchbase provides is the decoupling of disk IO and RAM. This basic concept allows us to provide extremely high performance at very low and consistent latencies. It also makes Couchbase capable of handling very high write loads without affecting the application's performance.
However, Couchbase still needs to be able to write data to disk and so 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. Information is written to disk through the disk write queue. The internal statistics system monitors the number of outstanding items in the disk write queue and can give you the information you need. The peak the disk write queue load 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 and 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.