The ver.req trick is no longer working on newer cameras. If it would, I think it would display the value of param
#1 as the number of total shots. Go to "miscellaneous stuff" -> "debug parameters", set "debug data display" to "params" and "propcase/paramsdata page" to 0. Leave the CHDK menu. The number will be displayed after "1:", you can verify it by shooting another picture and returning to playback mode.
It's possible that this number can be reset under certain circumstances, so I wouldn't trust it completely.