I think you knit 10 for the first two sizes, 7 for the third, and 10 for the 4th size. It is abut confusing. They do it for consistency though. In some parts of the pattern your size might be different.
What are you making? Always include as much info and a link if possible when asking questions. There are so many patterns it helps us answer better if we know what we're dealing with.
I'm not sure which size is medium but the first size is k10, the fourth size is k7 and the second, third (possibly medium?) and fifth sizes skip this instruction and go on to the next instruction.
If there's a 0 for your size, you get a free pass on that particular set of instructions and can proceed to the next set.