Counting plates works if real plates are used. Counting plates is bad if you are using paper plates---many people take 2 plates and double them up. The best lessons learned are the ones that hurt us the most----I remember taking one plastic sleeve of 125 Chinet three compartment plates to a dinner for 100 people---thinking I had plenty. Then I ran out of plates. I usually count silverware for a close estimate.
When I first set up at a dinner I usually count the tables and how many chairs at each table. If there is 15 eight foot tables with 8 chairs each then I know that there is seating for 120 people. Then right at the end of serving I can quickly look at the tables and estimate the crowd. If I am concerned with head count at a certain event--then I have someone count heads as they go thru the line.
The biggest problem I have had is when tickets are presold to a dinner and people forget to bring their tickets, and as a caterer you are getting paid for each ticket that is turned in. People can become real jerks when you tell them that a ticket is required----they know that they bought a ticket and from their viewpoint they don't see a problem. I make sure that one of MY people collects the tickets---not a member of the group that we are cooking for. I learned that lesson one day when I fed about 300 bikers and 100 tickets were collected----it got kinda ugly.
Caterers on this site talk about guarantees and deposits and payments etc------and all for good reason----we have all learned ugly lessons that hurt us.
Service Clubs that say "Our treasurer isn't here" The groom that left on his 2 week honeymoon to Hawaii with your check in his pocket. The big corporation that issues a "purchase order" instead of a check. The check arrives 90 days later. I have found that the largest corporations will screw you the fastest---to beware of governmental agencies----and a good ol' boy wearing bib overalls and shaking your hand is a sure thing. If you are catering for anything that has the words "Law Offices" in it----just bend over.