So far, only 13 states and the District of Columbia have told the Obama administration they intend to set up the insurance exchanges that are supposed to provide a marketplace for people to buy health plans. None are being watched as closely as California, whose singular challenges, from the size, diversity and geographic spread of its uninsured population to its vast budget problems, make it stand out. Many feel a successful rollout here could convince other states with high numbers of uninsured residents that the law can be made to work for them.