This verse essentially summarizes verses 19-20.

You must wash your clothes on the seventh day, and you shall be clean: It should not be implied that the soldiers had to use the water of purification, just mentioned in the previous verse, for washing their clothes. The seventh day indicates that their period of ritual confinement outside the camp lasted seven days (see verse 19).

And afterward you shall come into the camp: The Hebrew clause here expresses permission to enter the camp on the seventh day; it is not a command. So it may be rendered “and will be permitted to enter the camp” (Good News Translation), “Then you may return to the camp” (New Living Translation), or “afterward you can return within the camp” (La Nouvelle Bible Segond).

Quoted with permission from de Regt, Lénart J. and Wendland, Ernst R. A Handbook on Numbers. (UBS Helps for Translators). Miami: UBS, 2016. For this and other handbooks for translators see here .