How many JVM can be run on an operating system (OS) ?

You can have one JVM per process. Since an OS supports many processes, you can have many JVMs running.
When ever we start a new java process by invoking java.exe (i.e. java [class-name] ) a new instance of JVM is created. Each java process executes in its separate JVM environment – we can specify different JVM parameter for each process.
So you can open as many as command prompt your machine allows and run as many JVM as possible. It also means you can run different versions of JVM.