Combo orders consist of 3, 4, or 5 served dishes:
x3 combo order: 3 dishes were served—reward +2 coins per dish
x4 combo order: 4 dishes were served—reward +3 coins per dish
x5 combo order: 5 dishes were served—reward +5 coins per dish.
To receive the maximum reward, you must serve 5 orders within the time allotted on the timer. If you don't serve the next dish or have already completed a 5-dish combo, the timer will reset and you'll have to rack up combo orders anew.