How much???

Do a search for catering, etc. and you will get some good ideas. There are a few out there with nice spreadsheets that help you estimate the amount you need.

For buts, assume that you are going to get about 40-45% shrinkage in the meat during the cook and with the discarding of the bone and chunks of fat. That will be a fairly safe estimate for the resulting meat for serving. Then assume 1/4 to 1/3 a pound per serving depending on sammies or plates.

For ribs, you are usually OK to get 4 servings out of a rack of trimmed spares or 3 per rack of baby backs.

This is usually safe for vending, main meal stuff. If you are
doing a party where people are going to be grazing rather than sitting down to eat then you can probably go lighter.