Depends on where you live. In some large cities there is a maximum number of taxis allowed. They have get permits/medallions to operate. In these cities where Uber is not considered a "taxi" they can operate as many cars as they want.

I don't think that means it is a monopoly.

It may be more accurate to call it an oligopoly, but the distortion of the market from limited supply and competition remain the same.

I’d compare it to how ISPs are in the US. Most people believe ISPs have local monopolies which is why Comcast sucks. But we can’t simply change services because laws restrict ability for new ISPs to be started. Same issue with taxis. There are limited options and the medallion model limits the competition and new options from being created to compete.