As you know, brisket shapes and sizes are quite varied, so the first thing you must do, is to buy the pieces in groups of the approximate same shape and weight for each group. If there is much difference, the small ones will shred before the larger ones are tender enough to slice.

Oven or roaster roasting is a viable alternative to stock pots. Cooking in the oven, meat done a day or more ahead, chilled, sliced and reheated, is actually the easy way to do a big corned beef dinner.
Here is a compilation of many corned beef threads:

Basic guide:

www.ellenskitchen.com/forum/messages/8118.html

Here is a reliable selection of tested baked or oven roasted corned beef recipes:

www.cooks.com/rec/search/0,1-0,baked_corned_beef_brisket,FF.html

Electric roasters:

www.chefsline.com/blog/quick-tips/making-corned-beef-in-a-roaster/

Roaster slow cook is a great method for corn beef, double the cooking time and cook at 250. For three pieces, figure the cooking time on the smallest and add 1/2 hour. Keep the lid on- peeking lowers the temp enough that it increases the cooking time.

Corned beef is best when cooking slowly and with some moisture, more or less braising. Limit the number of times you open your roaster because heat will quickly escape. In an electric roaster, cook the corned beef brisket at 350 degrees; a large roast (6-7 pounds) will take about 5 hours. The cook time for the brisket will depend on your roaster and how often you open the door/lid.

The corned beef is finished when it reaches an internal temperature, measured with a meat thermometer, of at least 145 degrees minimum but it will taste best when 160 degrees and fork-tender.

Preheat roaster oven for 15-20 minutes before adding the meat.

Braising Liquid
Join foil strips, using a butcher wrap to seal the joins, make a piece that will line the whole roaster and wrap completely around the roast and future vegetables.
Place your brisket, fat side up, on overlapping sheets of aluminum foil. Place in roaster pan, then add enough liquid to cover 3/4-1 inch of the way up the roast, about 2 cups. For your liquid, use water, apple cider, apple juice, water with cider vinegar, wine, etc.; it really depends on what you like. Add some orange peels (white pith removed), whole cloves, and 2-3 whole bay leaves, or the spice packet that comes with the roast. Wrap roast by folding top edges together (so you can peak inside to see the roast easily).

After 4 hours when it’s around 140 degrees, drain most of the liquid into a pan- a turkey baster works well for this. Take out the meat, using forks or gloves, and set aside. Place small or quartered potatoes, large chunks of carrots, turnips or other root vegetables, onions if desired on the aluminum foil. Put the corned beef on top of the vegetables, fold the foil back over, and return all back to oven for about 40 minutes. Layer cabbage pieces or wedges all over the top, cover, cook 20-30 minutes until cabbage is tender.

If baking:
Here is the basic baked info:

4-6 pound pieces: Soak in fresh water to desalt, 1/2 hour or longer. Use heavy foil, place beef in the middle, add 1/2 cup fresh water, few slices onion, celery, carrot. Seal with butchers fold, so package is water tight. Bake in a slow oven (300 degrees F.) 4 hours. Cool slightly, unwrap and put in shallow pan. Spread with brown sugar mixed with mustard. Bake in moderate oven (350 degrees F.) 20 minutes or until glazed.

You simmer or roast the veggies separately, don't overcook the cabbage!

Reheating
I cover it tightly, small amount of cooking water in the pan, stick it in the preheated oven at 350 or higher, about 2 inches of meat per pan. Temp should be 180 in the middle when it is taken out to serve (about 1/2 hour).

Here are some other corned beef threads:

www.ellenskitchen.com/forum/messages/40926.html

www.ellenskitchen.com/forum/messages/40857.html

This amount of veg takes about 6 6 quart pots.

Vegetables
small red potatoes, 20-25 pounds. If larger, cut o more or less the chunk size of the carrots.
medium to large carrots, cut into 2-inch pieces, 10 pounds, Baby carrots are too small, unless you add them with the cabbage, later
30 celery ribs, cut into 2-inch pieces (about 2 large heads)
1 1/4 cups chopped celery leaves (from the celery above)
turnips OR parsnips, peeled and cut into wedges, 5 pounds
12 medium heads cabbage, cut into 6 to 8 wedges
36 ears fresh corn, halved (OPTIONAL, but very popular)
Broth:
10 12-ounce bottles Guinness stout or other stout or porter
10 cans beef broth OR strong vegetable broth (scant gallon)
20 bay leaves, crushed
8 tablespoon coriander seeds
10 whole allspice OR 40 whole cloves
OPTIONAL 8 dried chile de árbol, about 3 inches, hot
3 tablespoons black peppercorns, crushed
2-3 tablespoons mustard seed
OPTIONAL 6-8 sticks cinnamon
water to cover vegetables- amount depends on size and shape of pots

Bring the pots of liquid to a boil. Add potatoes, carrots, celery, celery leaves and turnips; return to a boil. Reduce heat; cover and simmer for 20 minutes. Add cabbage, beans and corn; return to a boil momentarily. Reduce heat; cover and simmer 15-20 minutes or until vegetables are tender. Yield: 70 servings.

Strain simmering liquid and chill or freeze strained broth for pea/bean soup base.

Wow, I found this page years ago when I first started my St. Paddy's Day party, thought I had everything down to a science but now that my party has moved past 4 burners (we are up to 40 people and 27lbs of corned beef) I'm starting to panic if we manage to increase in size any further.

I usually buy my beefs in 5-6lb cuts because the timing is nice and reasonable. If I put two 5lb cuts into one big pot to simmer (assuming I can find a pot that can handel it) am I looking at a time increase beyond 4ish hours? How do I account for that?

Allow an extra hour, but start checking at normal quit time, and pull when done to your preferred tenderness- I don't go to shreddy, some folks like that.

How long should I cook them, and should the meat be covered by water, or just enough water in the bottom to keep moisture in the oven?