The primary considerations when planning your Couchbase Server cluster are:
How many nodes do I need?
How large (RAM, CPU, disk space) should those nodes be?
To answer the first question, you need to take into account the following different factors:
Disk throughput and sizing
Data distribution and safety
Each of these factors can be the determining factor for sizing, although due to the in-memory nature of Couchbase Server, RAM is normally the most important factor. How you choose your primary factor will depend on the data set and information that you are storing:
If you have a very small data set that gets a very high load, you'll need to size more off of network bandwidth than RAM.
If you have a very high write rate, you'll need more nodes to support the disk throughput of persisting all that data (and likely more RAM to buffer the incoming writes).
Even with a very small dataset, under low load, you may still want 3 nodes for proper distribution and safety.
With Couchbase Server, you can increase the capacity of your cluster (RAM, Disk, CPU or network) by increasing the number of nodes within your cluster, since each limit will be increased linearly as the cluster size is increased.