So, the first choice you have to make is the type of potatoes to use. You don't want reds because they're not starchy enough. But yellows (like Yukon Golds) make wonderful rich mashed potatoes, much better than russets IMO.
Then decide what flavorings you want in them. Cheese? Garlic? Herbs? If you're going to be serving them with gravy, then you want something that will complement rather than clash, so stay away from flavors like blue cheese, but something like asiago or parmesan or even jack should work well. For garlic, probably roast it first, so it's mellow and not sharp.
I usually assume about 1/2 a lb of potatoes per person, minimum. I know a good sized baked potato can weigh a pound on it's own. But you can't really have too many mashed potatoes, IMO. I also never boil potatoes, because they absorb too much water. I prefer to steam them for mashed - they cook quicker (less water to boil), and they're not all waterlogged afterwards. Peel them or not as you prefer. I like the peels on and a chunky mash, but some people like smooth uniformly colored potatoes. That's totally personal preference.
As for what to stir in, butter and heavy cream give you that rich mouthfeel, but you also need something watery so they don't get greasy - that might be chicken stock, milk or water. Cream cheese lends a richness without the added fat some other stir-ins add, some people use sour cream for the tanginess. It's such a matter of personal preference (and what you have in the house).
If you won't be having gravy for some reason, then the sky is the limit. My personal fave for Thanksgiving is a mix of mashed potatoes and sweet potatoes with rosemary, garlic and parmesan. It wouldn't go well with most gravies, and if your family is into the marshmallow sweet potato thing, they may not go for it, but my family loves it.