As of March 11, 2009, there have been a total of 15,645 executions due to the death penalty in the U.S.; 14,489 of these occurred before the U.S. Supreme Court declared capital punishment unconstitutional in 1972, and 1,156 occurred after capital punishment was reinstated in 1976.
There is no definitive answer to the question of how many innocent people have been executed in the U.S.; however, Northwestern University School of Law's Centre on Wrongful Convictions (CWC) documented 38 executions carried out since the mid-1970s where there was compelling evidence of innocence or serious doubt about guilt. Another 130 death row inmates were exonerated, instead of executed, between
1973 and 2008 due to emerging evidence, including DNA analysis. A smaller number of people have been exonerated posthumously.
The Death Penalty Information Center estimates for every seven executions, one death row inmate is exonerated. Some of the prisoners they believe were innocent appeared to have developed dubious alibis late in the appeals process, so the determinant of guilt or innocence is subjective in many cases.
There are no records concerning wrongful executions, apart from where posthumous pardons have been granted or extensive and perseverant research has revealed a problem with the process. It isn't possible, generally speaking, for governments to admit they've killed someone illegally, so it is never discussed nor debated.