how many lbs of turkey or capon chicken should i buy or 8 people? we will also have ham?

The leftovers are the best, I have 8 adults and 2 children coming and I ordered an 18-20 lb bird.

I would go with a 15-20 lb turkey to ensure leftovers! The best part, no? At a meal about abundance, probably best to ensure that all 8 people are well-fed (despite the ham).