The amount of lobster you need per person is around 1 to 1 1/2 lbs of lobster per person.
So if you're feeding 5 people you should have at least 5 lbs of lobster to serve to those 5 people and 10 lbs of lobster for 10 people.
But you might want to aim for 2 lbs of lobster per person if the people eat a lot of lobster.
But if all you can afford is 1 lb of lobster per person that's okay too as lobster is expensive.
As long as you have at least 1 to 1 and 1/2 lbs of lobster for each person that you're serving you should be okay.