The MLA’s system of documentation is based not on publication format but on a template of core elements. For each slot in the MLA template, you should include the pertinent information provided by your source. If a magazine you are citing provides volume and issue numbers in addition to a date, include the volume and issue numbers in the number slot and the date in the publication-date slot:

Jones, Mel. “The Second Racial Wealth Gap.” Washington Monthly, vol. 47, nos. 11-12, Nov.-Dec. 2015, pp. 11-14.

Note that a different version of the same source might not provide information for all the elements. The online version of the Washington Monthly article provides only months and year, so you would not include volume and issue numbers:

Jones, Mel. “The Second Racial Wealth Gap.” Washington Monthly, Nov.-Dec. 2015, washingtonmonthly.com/magazine/novdec-2015/the-second-racial-wealth-gap/.