Luke 21:18

“And not a hair of your head will perish.”

But Jesus' final guarantee to His own was that not a hair of their heads would perish (compare Luke 12:7; Acts 27:34). This was not intended to be taken literally, indeed could not be, for the number of our hairs is not fixed, and when we lose them they perish. The saying was probably proverbial. The point was that at the resurrection every hair would still be in place, even though they had been burned or had been ground to powder or had decayed in the grave. Eternally they were totally secure. That is why they did not need to fear those who could only kill the body (Luke 12:4), but could not prevent every hair from surviving. This is confirmed by the next verse which also has in mind eternity.