Why is there no 'm' before this word? How would I no whether this is plural or singular?
Seremala is in the ji-ma noun class and not the m-wa noun class. Hence it's plural is maseremala instead of seremala, just like other nouns in the same class (ji-ma) singular|plural tunda|matunda (fruit|fruits) jitu|majitu (giant|giants) yai|mayai (egg|eggs)
the context would inform you of whether its a single carpenter being spoken of or multiple. e.g. seremala walikuja hapa/seremala alikuja hapa (the carpenters came here/the carpenter came here)
Seremala, maseremala was borrowed from Arabic sār-ˁamala ("foreman at construction work"), Malay sərimala and/or Malgasy soromala ("master of any art or profession"), according to the World Loanword Database.