Cooking Question (Roast Beef)

Once again, a little man has jumped into my body and volunteered me to have a large number of people at my home for dinner when I have less than zero time for it.

So, I'm thinking I will make roast beef since the vast majority of it involves putting it in the oven, and taking it out again. I can handle that.

My question is this: how many pounds of roast beef do I need for 10-12 adults and 5-7 kids?

Also, do you have any idea how long it will take to cook?

Thanks!