On the Canon 450D, the number at the far right of the viewfinder is the remaining burst count: how many more shots you can take in continuous shooting before the camera’s buffer fills up.

As you shoot rapidly, that number drops. It rises again as the camera finishes writing images from the buffer to the memory card. If you shoot single frames with pauses, you may never notice it change much, but in continuous drive it becomes more obvious.

So in your example, the “6” means the camera estimates it can capture about 6 more images before the buffer is full.