I think a lot of it depends on how each individuals style of hunting has evolved, and what sounds they feel the most confident using.
20 years ago I carried 5 or six calls with me, and I did a lot of the 20 min. on the stand then move kind of hunting. That was long enough ago that the dogs heard very little calling so they weren't nearly as call shy... I just changed calls so I could find out what worked best. I figured out the Crittr-Gettr worked the best for me... then after that it was the only call I took.
Once I moved to Gunnison, CO and there were guys out calling, I figured out I needed to change up the sounds now and again... so I learned how to use 'coyote talk'... still with the Crittr-Gettr, and killed quite a few dogs in areas that were well hunted.
Now... I'll sit in one spot for hours... glassing and throwing out a howls or a few pup distress calls then watching what happens. For the guys who call a lot... and move a lot... and hunt areas that are called a lot... you'll need a lot of calls. E-callers are cool... and effective... I just won't hunt with one, it takes a lot of the fun out of it for me. I'd rather kill one dog that I glassed up, than 3 that were called in by a machine.