Uber cars, often black sedans that can be summoned with smartphone apps, now outnumber the yellow taxis that city riders have hailed with a whistle and a wave for generations. [...] there are about 440,000 yellow cab rides a day, compared to just 20,000 to 30,000 Uber rides. "Uber having an unlimited number of cars means no drivers — taxi or black car and livery — will earn a decent living," said Bhairavi Desai, executive director of the New York Taxi Workers Alliance, an advocacy group that includes both Uber and yellow cab drivers as members.