Some scrips have circuit limits and they can be like 5%, 10% or 20%. You can view those limits on BSE website. Thats the limits it can vary on any given day, which is based on previous closing price. when it hits upper circuits people know that next day the demand might be more and hence won't sell, and vice versa. I beleive this limits are set by the exchanges to limit the operators activity, but in reality it helps them to manupulate the price.
It doesn't depend on no of shares traded on the limits(upper), thats the maximum price someone can buy or sell on that day(min for lower limits). Hope it helps.
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