Welcome to EpicSki, Watchout! As a long-time Summit County resident, and former instructor at Keystone (15 years), I agree with what the others have said here. Keystone can be as crowded as anywhere in Summit County, but mid-December is a very slow time, so it should not be too bad. Only if we have poor early season snow, in which case Keystone would not have a lot of runs open, should you expect to have any real crowd problems that time of the season--and then, only on the weekend. Do be ware of a couple bottlenecks near the bottom of the mountain, where several trails merge, especially at the end of the day.
Keystone is an under-rated resort, in my opinion, often wrongly maligned for lack of good advanced terrain. In fact, it has some of the longest sustained blue runs anywhere, arguably THE longest green run in Colorado (Schoolmarm), and once the snow fills in, some excellent if not deadly-steep bump and powder runs on North Peak and the Outback.
Unfortunately, and with regret, I no longer recommend the ski school at Keystone. For a variety of reasons, many of which I do not know (haven't worked there for five years), it seems that almost all of their top instructors have left for "greener pastures" (Vail, Copper Mountain, Breckenridge). I know no other ski school that has suffered such an exodus of talent, anywhere. If your party intends to take lessons, you might well want to consider one of the other nearby resorts.
Have fun, and let me know if there's anything I can help you with while you're here.