Depending on how much cucumber you'd like in your salad, you can use anywhere from 8 to 15 cucumbers for a salad serving 30.
Generally speaking, 5 lbs of potato salad will fee 30 people. 12 people would be a little over half of that, so around 2.5 lbs of potato salad.
about 4 whole roman lettuces
1 Gallon of potato salad feeds about 30 people, so 3.33 Gallons.
This depends upon portion size, the type of lettuce you and how many other ingredients are in the salad. Generally you decide on a recipe and, according to how many portions it will serve, multiply the amount of lettuce per portion by 70.
I would make 30 pounds
Generally speaking, 5 lbs of potato salad will fee 30 people. 12 people would be a little over half of that, so around 2.5 lbs of potato salad.
30
Typically a serving is about half a cup in volume and weight 4 ounces. There are about 4 servings in a pound of salad. That would mean about 7 pounds of salad to get everyone a single serving. If I had a picnic I'd probably get about 5 pounds of each.
It depends if you only serve pasta salad or something else with it. When you serve only the pasta salad - I would use 6 packages ( 6 pounds ) of pasta, 6 tin peas, 24 tomatoes, 8 onions, 2 pounds ham, 6 tin corn, 3 red pepper. If you want to serve this to a bbq - I would recommend a quarter of the recipe above.
12-15 pounds of potatoes should be sufficient. The remaining ingredients should be converted from your preferred recipe, based on the potato quantity.
A large head of lettuce should be enough to feed up to 6 people, depending on what else you are adding to it. For 30 people, about five heads of lettuce should suffice.