Most of answers, you know correctly.
1. How many people.
"두명인데요?" is okay but it's better to use "두명이에요."
2. Ordering menu
You can use '몇개' or '몇인분' or '서수(ordinal number)'
2짬뽕+1탕수육 => "짬뽕 두개랑 탕수육 하나 주세요."
1짬뽕+1짜장면+1탕수육 => "짬뽕 하나, 짜장면 하나, 탕수육 1인분 주세요."
3. Wrap up and check.
You know right. Just say it together.
"여기 계산해주시고, 남은음식 싸주세요."