The amount of swordsmen that Benjamin mustered depends on how one takes the second part of the verse. If it is translated besides the inhabitants of Gibeah, who mustered 700 chosen men
(ESV), then the Benjamites mustered 26,000 men. If it is translated apart from the citizens of Gibeah they mustered 700 chosen men,
then 25,300 men were mustered from Gibeah, and only 700 from all the cities of Benjamin. If it is translated apart from the citizens of Gibeah, who mustered 700 chosen men,
then the Benjamites mustered 26,000 men and Gibeah mustered 700. This seems to agree most smoothly with Joshua 18:21–28, where Gibeah is listed as one of the twenty-six Benjamite towns.1
15 And the people of Benjamin mustered out of their cities on that day 26,000 men who drew the sword, besides the inhabitants of Gibeah, who mustered 700 chosen men.