(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the אִישׁ term, by employing a situation-oriented construal as outlined in “Notes on Gender in Translation,” pp. 11–16.)
This usage of אִישׁ is conventional: it profiles the referent as a distinct entity within a larger military force, here under David’s unified command.
On the regular usage of singular אִישׁ as a “collective” term in the context of hostilities, see my comment to Judg 7:23. On its occasional use to denote a contingent of military forces, see my comment at Judg 7:24.