President Obama’s advisers are focusing on a strategy for Afghanistan aimed at protecting about 10 top population centers, administration officials said Tuesday, describing an approach that would stop short of an all-out assault on the Taliban while still seeking to nurture long-term stability. Mr. Obama has yet to make a decision and has other options available to him, but as officials described it, the debate is no longer over whether to send more troops, but how many more will be needed. The question of how much of the country should fall under the direct protection of American and NATO forces will be central to deciding how many troops will be sent.