I think if you are not a confident cook then making something that 'seems' simple like a roast can be quite stressful as there are so many elements to time and get right and you will be a bit distracted with 12 people milling about as you are trying to finish it off etc.
I am an experienced cook and I cook fairly regularly for dh's side of the family (approx 16) for Sunday lunch and other occassions and I would never ever consider cooking a roast on these days. I make life easy for myself and always cook something in a big pot with lots of nice sides, all of which can pretty much be made in advance. This takes the stress and exhaustion out of it for me. So for instance I have made the following over the past couple of years:
Chicken curry - made the day before, have chutneys, dips and poppadoms ready made and just have to cook rice on the day.
Lasagne - served with big bowls of salad with homemade dressing and roast garlic garlic bread
Pork stroganoff - made the night before and reheated very successfully and very delicious, I used a Jo Pratt recipe, cooked rice on the day and had a 'winter salad bowl' also a Jo Pratt recipe - delicious and hot crusty bread
Beef and red wine stew (bourguinon) made the day before and served with extra creamy mash made earlier that morning and popped into the oven with some butter on top and covered with a lid / foil, just stir the butter through when serving. Served this with steamed tender stem broccoli
Chilli con carne (nigella's recipe) served with bowls of cheese, gaucamole, sour cream, nachos and cook rice when they arrive
These were all winter dishes. I summer I have made things like:
Whole cooked salmon served with new potato and bacon salad, cucumber relish and bread
Chicken marinaded in honey and mustard traybake with potato gratin and green salad.
Thai Coriander Pork (Rachel Allen recipe) with rice
I don't do starters but sometimes stretch to a platter of small triangles of brown soda bread (i bake) topped with smoked salmon and pickled red cabbage with dill (buy the cabbage in M&S) OR olives, crackers and dips, especially if I know they won't all arrive together and we could be waiting about for one lot to show up etc. Saves them all wandering about and you having to offer endless cups of tea etc.
For dessert I always bake / make 2 things that can be made in advance so usually something like a chocolate cake or tiramisu or hazlenut pavlova etc so all I have to do is slice it up and serve.
Good luck, it is a lot of work, but if you play it clever you can minimise the effort on your behalf and still make something very impressive!
I thinks for big crowds it is nice to serve the main component (meat / stew etc) on the plate / dish per person (with some leftover for seconds) and then put everything else in serving bowls on the table or counter for them to help themselves - it creates more of an atmosphere of bounty and is less work than plating up everything for that many.