If you want one sandwich you can say "I would like a sandwich for lunch" If you want more than one (unlikely) you can say "I would like sandwiches for lunch" There are some foods that usually when you eat them you eat more than one. Then you would use the plural. Example: "I would like french fries" "I would like onion rings"

Do you want one sandwich or more than one? I would like a sandwich for lunch I would like sandwiches for lunch I would like a ham sandwich for lunch I would like ham sandwiches for lunch. I would like two ham sandwiches for lunch.