Un marks indefiniteness in the singular as well as plural (cf. un kasa, un ma kasa). Note that the feminine "Spanish counterpart" una is never used to mark indefiniteness (since gender is not expressed in Palenquero articles). Note, however, that as argued in Schwegler (2007), although common, articles in Palenquero are not obligatory (this includes un).
Source: Schwegler and Green 2007: 292,note 20