500
Having prepared himself well for the realtor exams and having exhausted everyone in the family with his requests that someone help him with the true-and-false drills, Jeffrey, who had never been a particularly good student in high school, knew he was ready to take on the greatest challenge of his life.
Trick question! This sentence is actually 100% correct.
Remember, just because a sentence is long, does not necessarily mean that it is a run-on sentence.