JVM
How many JVMs are available in a computer with dual operating system?
As many as the computer can handle... Usually, the JVM is started as a regular process on the host OS, so the same restrictions as for any other application apply.
A Java Virtual Machine runs as an APPLICATION on the host operating system. Same as ANY other application, if you install it on OS 1, it won't be available on OS 2 and vice versa. So if you have Linux and Windows, you have to install a JRE/JDK on BOTH systems.
As an execption, there are emulators to cross-run executables.In the "good old days" when there was no Oracle yet for Linux, you could still run on Linux the SCO version. There is even wine to run Windows applications on Linux. http://www.winehq.com/