To provide high availability and low latency for accessing a database, Cloud Spanner requires 1 node for every 2 TB of data in the database.
For example, if your instance has 1 database that stores 3.5 TB of data, you need to provision at least 2 nodes.
Those nodes will keep the instance below the limit until the database grows to 4 TB.
After your database reaches that size, you need to add another node to allow the database to grow.
Otherwise, writes to the database will fail. For a smooth growth experience, add nodes before this limit is reached for your database.