Scalable in which way? Horizontal or vertical? NoSQL DBs scale well horizontally, while RDBMS (SQL DBs) are scaled vertically.
Horizontal scaling (scaling out) means adding more nodes (machines) on which the data is partitioned. This can practically happen unlimited.
Vertical scaling (scaling up) means spreading the load between CPU and RAM on a single server (or a small group of servers), i.e., adding more power. There’s a practical upper limit to this.
There is no definite answer to your question. Both techniques involve trade-offs, for example in form of setup, management and maintenance costs, and complexity, availability, etc.