According to 1 Corinthians 15:6, the risen Christ appeared to more than how many brothers at once?

Answer: Five hundred

Paul lists appearances to Cephas, the Twelve, more than five hundred brothers at once (most still living), James, all the apostles, and himself.

“and that He appeared to Cephas and then to the Twelve. After that, He appeared to more than five hundred brothers at once, most of whom are still living, though some have fallen asleep. Then He appeared to James, then to all the apostles. And last of all He appeared to me also, as to one of untimely birth.”— 1 Corinthians 15:5-8 (BSB)