I second Pompeii. I've Used them several times and have been very happy. Ask about they're special stuffed pizza's. They make great party food because they are amazing and easy to eat with your hands. As for how much to order, this is what I would recommend for 20-30 people.
2 large tray's of the stuffed pizza(I recommend 1heart healthy and 1 'works') They can be cut in half and this alone will serve about 36 people.
1/4 tray of pasta salad
1/4 tray of the mela verde salad
you can also get forks/napkins included for a low cost. The menu and pricing is available online. Also, they are really good at helping you select the food and quantities!