3 LBS PER PERSON IS A GOOD GUIDELINE!
It really depends on your guests and if you are serving other food as well. Below is a good rule of thumb for your next crawfish boil.
These numbers are based on about 3 lbs. per person. This is an average order in a crawfish restaurant in Louisiana. Big eaters will eat 5 lbs or more.
10LBS
2-4 EATERS
15LBS
3-5 EATERS
30LBS
6-10 EATERS
60LBS
12-20 EATERS
90LBS
20+ EATERS