You're probably not going to like this answer, but I'm going with, "It depends." Think about what you're trying to accomplish with the fire you're building. Do you want low and slow with a long burn time? Do you want a lot of heat quickly, like when you're using the WSM as a charcoal-fired oven running at normal oven temps (325F+)?
If I were going to cook a whole chicken or a turkey, I'd be looking for 350-375F, so I'd likely start with a whole chimney of lit briqs. If I'm doing pork shoulder and shooting for 225F, I'm going to turn the chimney upside down and put maybe 10-12 briqs in the "bottom", dumping them all in the hole I've left in the center of the charcoal ring. Last time I did brisket I was shooting for 180F for the first five hours so I lit just five briqs and put them all at one side of the full charcoal ring. After five hours I jacked the temp to 275F for the rest of the cook. My approach worked well. Few lit briqs to start meant I could keep the temp very low. As it approached the five hour mark, enough briqs had caught that it was getting difficult to keep the temp down, but that was okay because I was about to want it hotter anyway. Last weekend I did Canadian bacon and again wanted a low heat, though this time for the whole cook. I made a two-briq snake around the perimeter of the charcoal ring and lit just three briqs using a propane torch. In retrospect, I should have gone for 225F instead of 180F. My plan was flawed, but my fire management fit the plan almost perfectly.
In addition to all that, you need to consider whether you're going to put water in the pan or not. If you're using water, then you want to start with more lit briqs. If you're running a dry pan then you don't need the extra heat to bring the water up to temp and can probably start with fewer lit briqs.
Finally, I would point out that it's almost always easier to move the temp up rather than down. If you start with too few lit briqs you just need to wait a little longer for the temp to come up to what you want. If you start with too many, you may overshoot your target and have to almost smother the fire to bring the temp back down.
