Those that were numbered of them, even of the tribe of Benjamin, were thirty and five thousand and four hundred.

Wesley's Notes

1:37 Thirty five thousand - The smallest number, except one, though Benjamin had more immediate children than any of his brethren, Gen 46:21, whereas Dan had but one immediate son, Gen 46:23, yet now his number is the biggest but one of all the tribes, and is almost double to that of Benjamin. Such great and strange changes God easily can, and frequently doth make in families, 1Sam 2:5. And therefore let none boast or please themselves too much in their numerous offspring.

Matthew Henry's Concise Commentary

1:1-43 The people were numbered to show God's faithfulness in thus increasing the seed of Jacob, that they might be the better trained for the wars and conquest of Canaan, and to ascertain their families in order to the division of the land. It is said of each tribe, that those were numbered who were able to go forth to war; they had wars before them, though now they met with no opposition. Let the believer be prepared to withstand the enemies of his soul, though all may appear to be peace.