Both "thread" and "threads" should be acceptable. In English it's unnatural to speak of "threads of clothes". Thread is also used when referring to the conglomerate of threads in a garment. "Do you have anymore thread like this? I need to sew a patch on this shirt." You speak about "loose threads" and "threads coming loose". People don't speak about "threads of clothes". So I'm wondering about the context in which Swahili speakers use "nyuzi za nguo". It would be interesting to know that. It would add a nice cultural element to our learning of Swahili.