How many people scuffle under the tree? Who does Sheriff Tate find has been killed in the scuffle?

The scuffle took place between Boo Radley and Bob Ewell. Ewell attacked Scout and Jem one night when they were coming home from a Halloween party. He was still hopping mad at Atticus for making him look like a fool on the witness stand during the trial of Tom Robinson, and like the coward he is, he decided to take out his anger on the Finch children. But Bob Ewell's in for a big surprise, as just after he attacks Scout and Jem, Boo comes to their rescue and in the ensuing scuffle, stabs Ewell to death.
The following morning, Sheriff Heck Tate finds Bob Ewell's body slumped beneath a tree; there's a knife stuck into his ribs. At first, it seems that Jem must've been responsible, as he was involved in a scuffle with Bob Ewell when he attacked him; Atticus is even prepared to defend his son in court if needs be. But it soon becomes clear that Boo was the one who killed Ewell and that he did so to protect the Finch children.