I would actually say 2 lbs of lox is way to much for 10 people unless it was all you were eating!
(or if you want the leftovers)
I usually do about 1/8ish lb pp if there is a "spread"
also if you are just going to do a fish tray- the place you order from will take care of all the calculations.
I'm in Deerfield, and Upper Crust Bagels does a great tray, as does Once Upon a Bagel in HP.
Further south, you have Kaufmans in Skokie, now back in biz, plus lots of other spots.
They will be happy to make it easy for you.
"If you reject the food, ignore the customs, fear the religion and avoid the people, you might better stay home."
~James Michener