1The amount someone speaks. 口数が多い means talkative; 口数が少ない means quiet or taciturn.
彼は口数が少ないが、言うことはいつも的確だ。
He doesn't talk much, but what he says is always on point.
お酒が入ると急に口数が増える人がいる。
Some people suddenly become talkative after drinking.
今日はやけに口数が少ないけど、何かあった？
You're unusually quiet today — did something happen?
2The number of mouths to feed — the number of dependents or family members one must support.
口数が多い家庭は食費のやりくりが大変だ。
Families with many mouths to feed have a hard time managing food expenses.
昔は口数の多い家ほど働き手も多かった。
In the past, the more mouths a household had, the more workers it also had.
3The number of lots, shares, or units (e.g., in stocks, insurance, lottery tickets). In this sense, also often read as こうすう.
宝くじを3口数買った。
I bought three lottery tickets (units).
投資信託の口数を確認してから売却した。
I checked the number of units in my investment trust before selling.