John Gill Commentary Numbers 2:16

"All that were numbered of the camp of Reuben were a hundred thousand and fifty and one thousand and four hundred and fifty, according to their hosts. And they shall set forth second." — Numbers 2:16 (ASV)

And all that were numbered in the camp of Reuben [were] an
hundred thousand and fifty and one thousand and four hundred and fifty ,
throughout their armies
151,450 men. Putting Reuben's host, and the hosts of Simeon and Gad together, (Numbers 2:13Numbers 2:15) , which was much lesser than the preceding camp, being 34,500 fewer in number:

and they shall set forth in the second rank :
they in journeying moving next to the camp of Judah, and before the tabernacle; for though, while encamped, the camp of Reuben lay on the south, or right side of it, yet, when marching, went before it.