Numbers 1:20
“And the children of Reuben, Israel’s eldest son, by their generations, after their families, by the house of their fathers, according to the number of the names, by their polls, every male from twenty years old and upward, all that were able to go forth to war;”
And they assembled all the congregation together on the first day of the second month, and they declared their pedigrees after their families, by the house of their fathers, according to the number of the names, from twenty years old and upward, by their polls.
As the Lord commanded Moses, so he numbered them in the wilderness of Sinai.
And the children of Reuben, Israel’s eldest son, by their generations, after their families, by the house of their fathers, according to the number of the names, by their polls, every male from twenty years old and upward, all that were able to go forth to war;
Those that were numbered of them, even of the tribe of Reuben, were forty and six thousand and five hundred.
Of the children of Simeon, by their generations, after their families, by the house of their fathers, those that were numbered of them, according to the number of the names, by their polls, every male from twenty years old and upward, all that were able to go forth to war;
Counter-Arguments
The strongest case that this verse does not belong in this theme.

Cross-References
“And it came to pass, when Israel dwelt in that land, that Reuben went and lay with Bilhah his father’s concubine: and Israel heard it. Now the sons of Jacob were twelve:”
“And these are the names of the children of Israel, which came into Egypt, Jacob and his sons: Reuben, Jacob’s firstborn.”
“Reuben, thou art my firstborn, my might, and the beginning of my strength, the excellency of dignity, and the excellency of power:”
“Every one that passeth among them that are numbered, from twenty years old and above, shall give an offering unto the Lord.”
“And Leah conceived, and bare a son, and she called his name Reuben: for she said, Surely the Lord hath looked upon my affliction; now therefore my husband will love me.”