There is video that I watched that showed me how to count objects. You can find that video on this very website. Clicking on 'Documentation' will get you to that video.

have somthing like: then do this every time a balloon get popped:

tylers solution will work if you want to internally keep track of how many balloons have been popped. If you want to visibly display a running counter showing how many have been popped, you will need to create or import a Counter class, link it to the class that detects balloon popping and control it from there. NOTE: if only one counter object is going to be in your world, linking is not mandatory. The Counter class should be able to create and update the image (as the count changes) and have public method(s) to adjust the count value and to return the current value.