It's difficult to determine exactly how many slaves escaped through the Underground Railroad. According to the Web site of the National Underground Railroad Freedom Center in Cincinnati, Ohio, "During the 1800s, it is estimated that more than 100,000 enslaved people sought freedom through the Underground Railroad" [source: Freedom Center]. Author James M. McPherson states in "Battle Cry of Freedom" that several hundred slaves escaped per year throughout the mid-1800s [source: McPherson], while the National Park Service Web site says that between 1820 and 1860, "The most frequent calculation is that around one thousand per year actually escaped" [source: National Park Service]. Another article in the Journal of Black Studies estimates that between 1830 and 1860, only about 2,000 escaped using the Underground Railroad. [source: Okur]
Historians do agree that, especially early on, most fugitives escaped from the border states -- Maryland, Kentucky and Virginia. Very few made it out of the Deep South, where conditions were often the most harsh, for multiple reasons. First, the journey North was much longer -- those who left usually went to Spanish-controlled Mexico or Florida. Second, when the government banned the African slave trade in 1808, slaves became much more valuable (due to a lack of supply). So in the Deep South, where the larger cotton plantations required more labor, masters were that much more inclined to control their "property." And finally, because slaves in the Deep South were farther away from the free states, they didn't have as much access to information about escape and what life was like in freedom.
To learn more about the Underground Railroad, take a look at the links on the next page.