How do I run the same workflow 100 times?

This depends. Why do you want to run it 100 times? If it's just a matter of looping you can implement this by putting your workflow as a nested workflow, and input to the workflow a list of 100 elements. (It doesn't matter what the elements are unless you are varying some parameter for each run.

For instance this Beanshell (with output port n) creates a list of the numbers from 0 to 99:

ArrayList n = new ArrayList();
for (int i=0; i<100; i++) {
    n.add(i);
}

If the workflow in question takes quite a long time to complete, you might not want to use this approach as it would mean the workflow would take 100 times longer to complete.

Instead you might want to execute the workflow outside Taverna, and use shell scripting to control the output and the number of times to run the workflow.