For actual numbers and graphs see this NYT article: https://www.washingtonpost.com/news/wonk/wp/2014/06/20/taxi-...
If you don't have time to read it, two important points:
- "In New York today, there are four drivers for every medallion, all but ensuring that an investor who owns one can find drivers to lease it 100 percent of the time."
- "The threat to medallion owners isn’t that they’ll lose passengers to these services. It’s that they’ll lose drivers — who have been aggressively courted by Uber. A taxi driver who doesn’t own a cab and medallion can earn comparable fares driving UberX passengers in his private car, without paying a lease fee."