WASHINGTON — Christmas at the White House isn't for sissies. Take quantities that might work in a private home – guests, cookies, parties, cards, whatever – and add some extra zeros to get a feel for a White House-sized holiday season. As in 50,000 guests, 28 parties and open houses, a couple hundred thousand holiday cards and untold quantities of cookies, cakes, brownies, truffles and the like to feed the Obamas' holiday throng. "They eat like crazy," says former White House executive chef Walter Scheib, who cooked for the masses under presidents Bill Clinton and George W.