Nikon d90 view finder problem?

In the viewfinder of my nikon d90, on the bottom right there's a symbol that looks like this [r21] I understand that when I shoot in continous mode, that number decreases as each shot i take with the camera. But why does that number vary sometimes. LIke when i turn the camera on, its [r5]. Why not [r21]?

See pages 65 and 262 in your manual. This is the image buffer size. It will change depending on how the camera is set up. Buffer size will be small if you shoot raw, larger if you shoot jpeg.