Jesus told the Twelve that anyone who wants to be first must be what?

Answer: Last of all and servant of all

After the disciples argued about who was greatest, Jesus said, "If anyone wants to be first, he must be the last of all and the servant of all."

“Sitting down, Jesus called the Twelve and said, “If anyone wants to be first, he must be the last of all and the servant of all.””— Mark 9:35 (BSB)