Clams come in different amounts per bushel of clams so it depends on the type of clams you buy as to how many people the clams will feed.
If you buy a bushel of littleneck clams there are 400 clams per bushel and if you buy a bushel of topneck clams there are 200 clams per bushel and if you buy a bushel of cherrystone clams there are 150 clams per bushel.
Then if you buy a bushel of chowder clams there are around 100 clams per bushel of clams.
Usually people will eat around 5 or 10 clams per person so you would take that amount and multiply it by the number of people to feed.
But lets say if your bushel of clams had 400 clams per bushel and each person you fed ate 5 clams.
If each person ate only 5 clams then you could feed 80 people off that 400 clams or bushel of 400 clams and if they ate 10 clams per person you'd need 2 bushels of those 400 clams to feed those same 80 people.