Last September, Uber’s top executives were pitched by some of Wall Street’s biggest banks, Morgan Stanley and Goldman Sachs. The bankers’ presentations calculated Uber’s valuation almost identically, hovering around one particular number: $120 billion. That was the figure the bankers said they could convince investors Uber was worth when it listed its shares on the stock market, according to three people with knowledge of the talks. Uber’s chief executive, Dara Khosrowshahi, and chief financial officer, Nelson Chai, listened and discussed the presentations, these people said.