How can I run a Minecraft Server on multiple CPU cores?

If you use the JDK instead of the JRE then you can use the following command line to force the JDK to use all available cores for optimising Java itself (source):

-server -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:+CMSIncrementalPacing -XX:ParallelGCThreads=7 -XX:+AggressiveOpts -Xms1G -Xmx8G -jar craftbukkit-1.4.7-R1.0.jar

As you are clearly using Windows, have you considered YAMS? (Full disclosure: I made it). YAMS will do all this for you.

Thanks for trying man, but a process in java needs specific triggers for a thread to split - it has to be coded that way.

You can get some practical examples here : https://beginnersbook.com/2013/03/multithreading-in-java/

While Minecraft has a decently robust developed multicore support on the client side, it lacks it on the serverside. Anything above 3 cores won't be used, unless new itterations introduces a command blockchain with automatical threadsplitting - making the dedicated server code more scalable than it's been since release.