My question is on the 2-3 sets in Stage 4 and 3-4 sets in Stage 5. I don't get what criteria we are supposed to use to decide how many sets to do. I admit that I may be missing something, but I honestly don't see the answer anywhere

. Is it something we're supposed to work up to or is it something like deciding you didn't work hard enough and probably can do another set?
The only thing I found confusing about the charts is that I thought there was supposed to be a warmup and then a warmup set for each exercise. Other people here have said they only do a warmup and then a warmup set for the first exercise, then continue on with the workout as written.