is ServletContext one per web-app or one per JVM ?

Each webapp will have its own ServletContext.

There is one context per "web application" per JVM

That means each web application has exactly one ServletContext on each VM.

Example

Several Web Apps on one Server (one JVM) : 

Web App A - Server/JVM 1 - Context 1

Web App B - Server/JVM 1 - Context 2

Web App C - Server/JVM 1 - Context 3

Same Web App on several Servers (several JVMs) : 

Web App A - Server/JVM 1 - Context 1

Web App B - Server/JVM 1 - Context 2

Web App C - Server/JVM 1 - Context 3

Web App C - Server/JVM 2 - Context 4