NET Bible inserts a heading here (“Purification After Battle”), but since the larger subsection covering verses 13-24 (or 12-24) deals with various issues involving “purification,” a heading is probably not necessary or helpful at this point.
Encamp outside the camp seven days: Moses tells the returning Israelite soldiers not to enter the camp for a week. If Encamp outside the camp gives the wrong suggestion that there should be another camp, separate from the main camp, it is better to say “stay outside the camp” (Good News Translation, New International Version, New Living Translation). NET Bible is similar with “remain outside the camp.”
Whoever of you has killed any person, and whoever has touched any slain refers to any soldier who killed a Midianite or who touched a dead one on the battlefield. Good News Translation places these two clauses near the beginning of the verse for naturalness in English, saying “all of you who have killed anyone or have touched a corpse.”
Purify yourselves and your captives on the third day and on the seventh day: These instructions follow the regulations in chapter 19 (especially verses 11-19) regarding purification from ritual contamination through contact with a corpse. The soldiers became unclean by killing people or touching corpses and had to go through a ritual of separation and cleansing (so Olson, page 179). In most languages the expression for ritual “purification” in chapter 19 can be used here (see the comments on 19.12). Good News Translation translates your captives as “the women you have captured,” since all the Midianite men had been killed (verse 7) and the boys were to be killed (verse 17). However, it will be better to say “all the virgins you have captured,” since all the other women were to be executed (verse 17). These girls must also be purified because they have presumably contacted dead bodies (so Gane, page 770). Good News Translation places on the third day and on the seventh day at the beginning of this clause, which other languages may find helpful.
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 .