Why was the tribe of Levi not counted together with the other tribes?
Rashi #1: Because it is correct for the king's legion to be counted independently.
Rashi #2: Because Hashem foresaw that all those who were counted from 20 years and upwards were destined to die in the desert, so He precluded the Levi'im from that counting, 1 because, since they did not sin by the Eigel, they belonged to Him. 2
Rashbam: Because they were not conscripted to go to the army like the other tribes. 3 Riva
Why does it say "Ach"?
Moshav Zekenim: "Ach" is an exclusion. Leviyim sometimes died due to the Aron (if they did not carry it with the proper Kedushah), so they were the smallest Shevet. 1
Seforno: Even though I commanded to count all of Yisrael, and Levi is included, do not count Levi like the others (from the age of 20), and do not include them in the total number of people.