I use the 3# bags from Sam's (Costco, BJ's, GFS, are all fine) I think 3# serves a recommended 16 servings. I always have coleslaw left over.
The dressing is tough - I make my own with mayo, vinegar, and Splenda. You'd probably need a couple of gallons of dressing. Make the slaw up by the big bowl or service pan, as needed instead of all at one time - that way is stays nice and crisp. Good luck.