Charles Spurgeon Commentary Numbers 8:7-8

"And thus shalt thou do unto them, to cleanse them: sprinkle the water of expiation upon them, and let them cause a razor to pass over all their flesh, and let them wash their clothes, and cleanse themselves. Then let them take a young bullock, and its meal-offering, fine flour mingled with oil; and another young bullock shalt thou take for a sin-offering." — Numbers 8:7-8 (ASV)

And thus shalt thou do unto them, to cleanse them: Sprinkle water of purifying upon them, and let them shave all their flesh, and let them wash their clothes, and so make themselves clean. Then let them take a young bullock with his meat offering, even fine flour mingled with oil, and another young bullock shalt thou take for a sin offering.

There are still, typically, these three things in the cleansing of God's people: the blood, the water, and the razor. There is blood, the emblem of the putting away of sin by Christ's atoning sacrifice; the water, typical of the Holy Ghost, by whom the power of sin is overcome; and then that razor, cutting off what grows of the flesh; what was their beauty and their glory is all taken away from them. There are some of God's people who have not felt much of that razor; but if they are to serve God perfectly, it must be used.

Let them shave all their flesh.