Agree with previous post - around 130 ish portions from a 60kg carcass - we get a bit more as we make home made stuffing too. Dont put anything on the skin and you should get fab crackling. That said we only do our own family one's as it a lot of work to cook on site, keep checking it, worrying about the gas blowing out, carving it up etc. Unless you are doing it regularly it's a heck of a faff and very time consuming. Real genuine free range outdoor roasted pork tastes fab tho' . Some of the one's we have tried that are at do's are tasteless and watery and cannot believe they get away with selling them!
Good luck whatever you do!