I don't know of any limit. I use 3 JDK's on this machine here. My employer has a legacy application which uses JDK 1.3, an application which is deployed with JDK 1.4.2 and I am experimenting with JDK 1.5.0.
There is no limit on the number of JVMs you can have. In fact if you have several IDEs installed on your machine, each one mostly ships with it's own JVM. You can configure them to use either their default ones or the JDK that you like.