I do a 4 day show every year that has about 14,000 attendance. That's the biggest I've done. We are usually very busy for a few hours each day. People waiting in line to pay, people with questions, people that want to see something in another color, someone who wants me to make a chain in a particular size. My daughter is a great sales person and she is always at my booth with me. We have a system. She handles the cash register, the cc charges, the packing and bagging. I handle the customers. When either of us has a spare minute, we help the other. I have a double booth space at this show.
During the really busy hours, I have my husband sitting in the corner behind my booth. He just sits and watches. Sometimes he'll talk about the weather or something. He doesnt do sales. I've had one thing stolen at a show but got it back as a crafter saw the person take some stuff and put it in a bag.
Make sure your money is secure. Have at least one other person to help you. 2 if you can do it for a show that big. Get things ready ahead of time. We have all our packaging in compartments in a box right at the pay station so we just pull out the size bag needed, info card, business card. Be organized and know where your stuff is so you can quickly get to it. Don't be looking away or lost in some box somewhere. Thats a perfect opportunity for something to disappear.
I have a friend who sells dips and she is always super busy. I was shocked to see her just throwing her money into a big box under the table, no organization to it. Her tables are up on blocks so they are higher than countertop and she was totally enclosed so it worked for her. I'm not suggesting you do that. Just an example that different things work for different people.
It's best to use a counter height stool rather than a chair. It's easier on you and you have more visibility of your stuff. Don't forget your counterfeit bill pen. We always use them to check large bills. Good luck at your show. Come back and tell us how it goes for you. We can all lear from other's experiences.