According to this article[0] (that someone referenced below):

A Cloud Spanner “node” isn't a physical machine, per se. It is an allocation of compute capacity and disk access throughput on a set of machines with access to the storage API.

(disclosure: I work at Google Cloud but not on Spanner)

[0] https://quizlet.com/blog/quizlet-cloud-spanner

They don't appear to document this, save that they handle 10k QPS read or 2k QPS write and can deal with 2TiB of data. They also recommend that you have 3 of them:

https://cloud.google.com/spanner/docs/instance-configuration