Elasticsearch can run on a single node and easily scale out to hundreds of nodes. It is very easy to start a single node instance of Elasticsearch; it works out of the box without any configuration changes and scales to hundreds of nodes.
Horizontal scalability is the ability to scale a system horizontally by starting up multiple instances of the same type rather than making one instance more and more powerful. Vertical scaling is about upgrading a single instance by adding more processing power (by increasing the number of CPUs or CPU cores), memory, or storage capacity. There is a practical limit to how much a system can be scaled vertically due to cost and other factors, such as the availability of higher-end hardware.
Unlike most traditional databases that only allow vertical scaling, Elasticsearch can be scaled horizontally. It can run on tens or hundreds of commodity nodes instead of one extremely expensive server. Adding a node to an existing Elasticsearch cluster is as easy as starting up a new node in the same network, with virtually no extra configuration. The client application doesn't need to change, whether it is running against a single-node or a hundred-node cluster.