But if they blow only one is literally “And if on one [trumpet] they blow.” But renders well the Hebrew waw conjunction here, since this verse contrasts with the previous one by referring to blowing only one of the trumpets instead of both of them. For they blow, see verse 3.

Then the leaders, the heads of the tribes of Israel, shall gather themselves to you: As in 1.16, the Hebrew word for leaders is nasiʾ, which refers to a prominent and distinguished leader, for example, a much respected sheikh of a tribe; thus leaders seems a rather flat translation. A better rendering in English is “chiefs” (Revised English Bible) or “chieftains” (New Jewish Publication Society’s Tanakh). The Hebrew phrase rendered the heads of the tribes of Israel is translated “the heads of the clans of Israel” in 1.16 (see the comments there). The Hebrew word for tribes is literally “thousands.” This word is not a kinship term, but refers here to divisions in a tribe, more specifically, to military units, so Levine says “militias.” Although Revised Standard Version translates the heads, there is nothing in the Hebrew construction here that necessarily corresponds with the definite article the. So the leaders, the heads of the tribes of Israel is more accurately rendered “the chiefs, heads of divisions in Israel.” It is possible but not certain that these chiefs were the only heads of this kind in Israel’s military organization. Good News Translation says simply “the leaders of the clans,” which is not quite exact, but it may be the best solution if a more specific rendering would turn out to be awkward or unclear. Shall gather themselves to you implies that these leaders will meet Moses at the entrance to the Tent of Meeting (see verse 3).

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 .