כַּאֲשֶׁ֛ר צִוָּ֥ה יְהוָ֖ה אֶת־מֹשֶׁ֑ה וַֽיִּפְקְדֵ֖ם בְּמִדְבַּ֥ר סִינָֽי׃ (פ)
여호와께서 모세에게 명하신 대로 그가 시내 광야에서 그들을 계수하였더라
Rashbam on Numbers
ויפקדם במדבר סיני, the reason why the Torah mentions the location of this count is because during the second such count in Numbers 26,3 the Torah mentions that it took place in the wilderness around Moav, prior to the assault on the land of Israel before crossing the Jordan river. At that time after the death of Aaron, Moses issued these instructions to Eleazar, Aaron’s son and successor.
Rav Hirsch on Torah
V. 19. ויפקדם במדבר סיני durch diese Wiederholung ist vor Beginn der Zählung nochmals Begriff und Bedeutung derselben in Erinnerung gebracht (siehe zu V. 1).
Chizkuni
כאשר צוה ה' את משה, “as the Lord had commanded Moses;” these words belong to verse 18, where Moses is reported as having assembled all the people. [I suppose our author found a sentence commencing with “as, כאשר,” as somewhat unusual. Ed.]