Corned beef from the round is sad. Avoid that even though it's readily available. Flat or point will cost you more but are worth it imho. My Costco has whole corned beef (both point and flat in one chunk) in cryovac ready to cook, but that's not so common. Most often it's just the flat. If you want to lean it out some, the flat is the leaner of the two cuts and easier to trim.
I prefer to pressure cook the corned beef. It's just convenient and fast. Or you braise it slowly in the oven or stove top. For firemen, probably figure 1/2 pound per person prior to cooking. More wouldn't hurt. You'll have a fair amount of shrinkage in corned beef from raw plus firemen are stereotyped as heavy eaters. Save some trimmed hard fat and render it for cooking the hash.
Cube up your beef--1/2 inch cubes would be my target, Cube up an equal amount of cooked potato--boiled is simplest. Onion, to taste. I'd go with green onion for color and it's somewhat "fresher" impact in food compared to a yellow or white onion.
For 12, I'd say you'll need two 12" cast iron skillets. Pan type isn't essential, I just like what cast iron does for crusting corned beef hash.
Heat the cast iron pans. Add rendered fat when hot. Add potato, season with salt--go a bit light to account for saltiness from the beef. Add corned beef, season everything with black pepper, stirring to combine Cook until it gets crusty. Turn, breaking up the crust. Repeat until everything is evenly crusty. Add chopped yellow onion for the final two crustings, probaly last 5 minutes of cooking. If using Green onion, only the last crusting is needed. Time will depend some on how full you pack your skillets. Give it a taste for seasoning and correct as needed.
It will hold well in a low oven while you cook the eggs. I recommend "basted" or steaming the fried egg. It's the easiest way to cook many eggs at once to the same doneness. Pre crack the eggs into a largish bowl, about 4 per bowl. 2 bowls per 12" skillet. That way you can add the eggs quickly to the pan so they have the same cooking time. On a home stove, you might have to do this in a few batches to serve more than 1 egg/person for 12 people.
The uncouth heathen in me likes crusty corned beef hash with ketchup.