Only 14 Americans who were once on death row have been exonerated by DNA evidence alone. ... In the Winter 2005 Journal of Criminal Law and Criminology, a group led by Samuel Gross, a law professor at the University of Michigan, published an exhaustive study of exonerations around the country from 1989 to 2003 in cases ranging from robbery to capital murder. They were able to document only 340 inmates who were eventually freed. (They counted cases where defendants were retried after an initial conviction and subsequently found not guilty as "exonerations.")This is an extremely small rate of exonerations, and the rate is likely to go down in the future. Only in the last 10 years have DNA exonerations been possible because the DNA evidence was not available at trial.
Common sense would indicate that many more innocent people are convicted. Human beings are fallible. Judges, juries, prosecutors, cops, and others have various biases. But there is no hard data showing that a significant number of people are falsely convicted.