You can usually expect lots of people. Collectors and book scouts are in line before the doors open. The experienced book sale buyer knows that the highly collectible books are often sold in the first hour or two of the sale. Remember that the sales are nearly always staffed by unpaid volunteers, so be patient with them. If you don't get to the sale when it opens, don't despair.
Many good books are available throughout the sale.
At some sales, donors bring books throughout the sale so you never know what will be there in the next hour. For the entire duration of the sale, you can always get good self-help books, mysteries, novels, and just about anything else you want. Book sale organizers often reduce the price of the books near the end of the sale. At that price, you can afford to bring a bunch home for friends to browse. Any that you don't want can be donated back next year.