Whether heads or cases, always figure it on weight. Just a lot easier.
If you are doing a 4 ounce serving, then 4 ounces from 16 ounces gives you four servings per pound of finished product. Assume some numbers...
Creamy slaw dressing is maybe 1/2 ounce of the total weight
Cabbage loses about 10% of the weight to water loss, so 3.5 x 1.10=3.85 ounces of veggies needed per serving.
So, you need 3.85 ounces of cabbage, and any other veggie, in total. This ends up being just under 4 servings per pound of slaw mix. Those are the numbers. Now...I just did en event, with 20 people, and 3 pounds of slaw. I would bet there was 1/2 pound left at the end of the event. So, self-serve, people served themselves less than 4 ounces per serving, something to consider.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."