How much beef and seafood should I buy for mixed 12 adult dinner party?

The rule of thumb for a standard dinner is:

• 250g of protein
• 200g of carbohydrates
• 200g of vegetables

for a man and:

• 200g of protein
• 180g of carbohydrates
• 200g of vegetables

for a woman.

But for a small dinner (<50 people) cook for one more person than is attending.
_{(Better be safe than sorry)}

As you're having a dinner party (which normally takes longer than a standard dinner) add 33% overall for an extended dinner party, which brings us to:

• 4 Kg of protein (8.5 pounds, to be split between Beef / Shrimp / Scallops)
• 3 Kg of carbohydrates (7.2 pounds)
• 3.5 Kg of vegetables (7.6 pounds)

for 6 women and 7 men for an extended dinner party. (13 total: one more than you need)

_{Note: Feel free to recalculate according to your needs. (E.G. if you have appetizers, soup, starter, dessert as well, don't use the 33% at all)
P.S. Sorry, but I cannot calculate in Imperial mass units, though I can convert the end result... 0:-)}

With meats, the general rule of thumb is 1 pound per dinner guest, especially when using a whole roast, turkey, or something similar that requires carving. Assume 2 scallops per person on average, as not everyone will be interested in this type of shellfish.

As for shrimp, I always refer to the table found on this page: https://farmtotableasiansecrets.wordpress.com/2007/12/17/medium-large-jumbo-colossal/ as it has always been accurate for me in the past. Thus, for your size, I would suggest an average of 5 shrimp per person.

Hope your dinner party goes well!