Jamieson, Fausset and Brown on 2 Corinthians 10:12

12. "We do not presume (irony) to judge ourselves among, or in comparison with, some of them that commend themselves." The charge falsely brought against him of commending himself (2Co 3:1; 5:12), really holds good of the false teachers. The phrase, "judge ourselves of the number," is drawn from the testing of athletes and senators, the "approved" being set down on the roll [Wahl].

measuring themselves by themselves--"among themselves": to correspond to the previous verb, "judge ourselves among them." Instead of measuring themselves by the public standard, they measure themselves by one made by themselves: they do not compare themselves with others who excel them, but with those like themselves: hence their high self-esteem. The one-eyed is easily king among the blind.

are not wise--with all their boasted "wisdom" (1Co 1:19-26), they are anything but "wise."