That's not exactly true, cities can and do still have limits on the number of cars, case in point Seattle:
hhttp://blogs.seattletimes.com/today/2014/03/seattle-city-cou...
hhttp://blogs.seattletimes.com/today/2014/03/seattle-city-cou...
Doing so is easily done as a consequence of Uber being centrally controlled, with full tracking on all its cars. A city can trivially know exactly to the minute how many cars are in operation via Uber at one time.
However, regulation of that sort isn't what Uber wants, they want a situation where supply is unbounded and people only compete/control demand.