I have not much experience with descr_rebels, but yes, one line of unit X means one unit of this type in a rebel stack of this particular style. You´ll no doubt have noticed that identical rebel stacks keep popping up time after time after time, that´s why.
The chance figure (is it given for each stack seperately?) I think gives the propability of this particular stack appearing every time a rebel group is created, whereas brigand_spawn influences the chance of whether or not a rebel stack of any type is created. So to change the chance doesn´t change whether or not rebels appear in the first place, it merely controls which kind of rebel stack is most likely to appear.
At least, this is how far my understanding of it goes.
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