I actually made a report about this because I wasn't sure, but now I think I was wrong. According to dictionaries, it always has "ma".

There are quite a few apparently singular words which always go in the "plural ma class", such as mazungumzo, mapenzi and the names of a lot of liquids (maji, maziwa, mafuta). For this reason, I think numbering words by class rather than referring to singular and plural is more accurate. The words with the ma- prefix are class six. Most class 6 words are the plural form of a class 5 word, but not all. Some are liquids, some are abstract, some are plurals or collectives of other classes such as, I think, class 14 words on the rare occasion they have plurals.

One other benefit of the class numbering system is that it's consistent across Bantu languages. Many languages have lost some of the original classes. 12 and 13 are lost in Swahili and I think they went higher than the 18 that Swahili has.