How many witnesses did Atticus call for the defense of Tom Robinson?
While the prosecution calls three witnesses -- sheriff Heck Tate, "victim" Mayella Ewell, and her father, Robert E. Lee "Bob" Ewell, Atticus calls only one witness in defense of Tom Robinson. That witness is Tom Robinson himself.
After being sworn in, Tom testifies that he had walked by the Ewell property on several prior occasions and had previously helped Mayella with chores upon her request. On the day in question, Tom testified, Mayella asked him to come inside to fix a door (and, later, to get something off of the top of her dresser). According to Tom's testimony, when he reached for the item, Mayella grabbed at his legs and, when he turned around, kissed him. Her father saw the kiss through an open window and called his daughter a "whore," threatening to kill her. This testimony is drastically different from the rape allegations presented by both Mayella and her father.
Likely, Atticus did not feel that he needed multiple witnesses, as his cross-examinations of the prosecution's witnesses led credibility to Tom's story. Mayella's account of a brutal attack, for example, is made less likely when Tom reveals that his left hand is useless, having been caught in a cotton gin years before. Likewise, the fact that Bob sought no doctor for his daughter after her alleged rape detracts from his story. While the verdict does not match the presentation, Tom's testimony shines as honest and forthright in contrast to the Ewells.