if you really want to use the character"口" you could say"我家里有5口人"=“我家里有5个人” wo3 jia1 li3 you3 wu3 kou3 ren2

yes, 口 is a measure word for people in the family

口 has other meaning . It means mounth and entrance. like 口腔 入口 oral cavity entrance and you can also say 我口渴了 I am thirsty

你家里有几口人啊？ 我家有五口人。 一口井

yeah, you can.just say 我家有3口人。“口”is a quantifier in here.