"About" and "around" mean the same in this sentence; they are common and used in casual face-to-face speech. "Roughly" might be used to emphasize the fact that you are just guessing the number of people on the bus, that the number is not accurate. "Approximately" means the same, but it is more of a formal, technical word that you would maybe use when writing a paper or legal statement.
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AnonymousYou wouldn't use that first sentence at all.Why can't I use more or less than? In what kind of situation can I use it?