number of words a person speaks
number of dependents; number of mouths to feed
number of shares, items, applications, etc.
彼は授業中、口数が少ない。
He speaks little during class.
悪い知らせを聞いてから、同僚の口数がめっきり減った。
After hearing the bad news, my coworker became noticeably quieter.
彼女は口数が少ないが、好きな話題では多くなる。
She is usually quiet, but when it comes to her favorite topic, she becomes more talkative.