When Cyrus is making his speech to the gangs, and explaining his plans for taking over the city, he says, "You are standing with 9 delegates from a hundred gangs, and there are a hundred more. That's 20,000 soldiers." Well, 9 times 100 is 900. Another 900 is 1800. Even if you round off to the nearest thousand you only have 2000, not 20,000.
Look at the quote. Nine delegates from 100 gangs. At this point, we have 100 gangs mentioned (all of them represented by the nine delegates). Then add 100 more and you have 200 gangs. There is no mention of how many people are in each gang, no average headcount, nothing. Some gangs may have five members, others may have 150, but this is an unknown. All you know from the quote is that there are 200 gangs being discussed. If he says that translates into 20,000 "soldiers," so be it. Have to take his word for that. There is no way to check his math. This is not a slip-up.