It may seem like a simple thing, but buying chips from a fish and chip shop is actually a complex game with rules that no one has ever really defined. They're certainly not sold by weight or volume or number. $6 worth of chips is not twice as many as $3 worth. So how do you get the right amount?
Firstly, you never mention the price. Buying chips by the dollar is a sure way to get less than you want. $3, $4 or even $6 can all be around the same amount of chips, because no one ever weighs or counts or measures them, so no one actually knows what they're worth.
Step one is to place the rest of your order. This gives the person serving you an idea of how much food you're going to require, but it's only half the story. They know what you've ordered, but not how many people are going to be eating it. Buying two pieces of flake, two dim sims and some chips for one person is quite different to buying the same for two.
So once they've taken down the rest of your order and can see how many "major items" (fish, souvlakis, burgers) and "minor items" (scallops, dim sims, potato cakes, etc.) you've got on the list, you need to let them know how many people you're trying to feed.
You can't just come out and say that though, that's not how it works. It just makes you sound like a weirdo. Why are you telling this person how many people you're sharing dinner with? Crazy.
The correct procedure is to ask for "about enough chips for X people" (where "X" is the number of people, obviously). Using the information they've gleaned from the rest of your order, the person serving you will work out how much food each person is likely to have assigned to them and give an amount of chips they feel complements that food.
What you'll end up paying is pretty much arbitrary, as it depends on the mood of the person serving you, but you can reduce the cost by being friendly and cheerful. As there are no fixed prices, they can charge low or high depending on whether they like you or not, so it pays to be nice.
This way you avoid coming home with a single serve of chips to share between four people that you paid six dollars for.
Of course, some places sell chips in a box and state clearly how much you'll get for what price. They just don't understand how the system is supposed to work.