Answer:
Twice.
Step-by-step explanation:
Thomas Malory's "Le Morte d'Arthur" tells the historical story of King Arthur and his knights of the Round Table. The story delves into the many relationships between the characters, and chivalry, quest, competition, revenge, etc.
Sir Gawain and Sir Lancelot are two of the Knights of the Round Table under King Arthur. Gawain and Lancelot first fought after the latter killed the former's brothers, Gareth and Gaheris. In his want for revenge, Gawain tried to entice Lancelot to fight him. But considering their previous closeness, Lancelot refused to fight him. This only made Gawain angrier, publicly accusing Lancelot of treason. So, Lancelot decided to fight him to defend his honor where he won but refused to kill Gawain.
After three weeks, Gawain challenges Lancelot again, and this time, Lancelot 'defeated' Gawain but again refused to kill him.
Thus, the two knights jousted twice.