Thanks for the kind words everyone!
Brendan - The number of shots I take depends totally on the subject (and keep in mind that this is only the second stacked image I've processed - so I have plenty of fine points to learn). With the RRS focusing rail I can make minute adjustments to the focus. I start by backing off enough that nothing is sharp and then use the focusing rail (it has a threaded bolt-type mechanism that moves the camera forward or backward when you turn a knob) to move in until the closest thing in the image (in this case, it was the tip of the swimming leg on the right side of the frame) comes barely in focus. Then I take a photo, give the focus rail knob about 1/2 turn, take another photo - and so on until the farthest thing away from me that I want to be sharp comes into focus. Pretty simple, really - with the caveat that the subject can't move during this whole process. Another point to consider is your f/stop - clearly, you would need to shoot more images at, say, f/2.8 than at f/16.