How many police cars are allowed to show up during a routine traffic stop? ?

Don't you hate those robotic answers: "If you have nothing to hide, you have nothing to worry about." If that were the case, we wouldn't need no Constitution to protect our rights, nor courts nor lawyers. Back to your question. There is no policy or regulation about the number of cars. You might make a lawsuit in civil court about intimidation, but to do so, you'd have to prove intent. Not just allude to it, but prove indisputably that's what they were doing. And since intimidation is the effort to coerce you into doing something you don't legally have to do, you'd be hard put to make your case. Police presence, by itself, does not constitute intimidation (in spite of what you see on TV dramas).