I've camped at Furnace Creek campground several years around Christmas/New Year's and have never seen it more than about a quarter full. I've never had reservations. However, several of those years others have told us about a Jeep get together that we just missed that had the place packed. Whether that means full or not I don't know. But apparently this is a yearly thing for these Jeep enthusiasts. Also, if going to Furnace Creek be prepared for a chorus of generators. If that's not your thing, check out Texas Springs campground, where they are not allowed (edited - I originally said Sunset but was mistaken).
If you are considering backcountry camping, there are
restrictions on where and how you can do that.
DV is a great place but it's BIG and attractions are spread out from each other. Be prepared for a lot of driving to see everything it has to offer.