You could certainly ask for one chip, but I'm not sure why. You wouldn't ask for one nacho.
"Nachos" are a dish that includes tortilla chips, but they are not the same thing. In the most basic form, nachos are chips with a spicy cheese for dipping. Jalapeno peppers are sometimes an option. Alternatively the cheese is already poured over the chips.
Basic nachos:
From there, the dish becomes more elaborate, often called "nachos grande" or "nachos supreme" or something like that. Then the chips would be topped with cheese, jalapeno peppers, taco-flavored ground beef, sour cream, etc.
More elaborate nachos:
To make it more confusing, there are also pre-flavored tortilla chips in "nacho" flavor, which are coated with a powdered flavoring meant to simulate the flavors mentioned above (namely cheese and pepper). "Doritos" was the original brand name.
Nacho cheese-flavored chips: