I didn’t know that you could only order nabe for two or more people.

For cuisine such as yakiniku or nabe (hot pot), where people share the dish, or for course meals, even if the menu lists the price per person, there may be a two-person minimum order. The menu might display a photo of the meal, but often the photo shows the volume shared by a certain number of people. Basically, you can’t order something not shown on the menu. However, the restaurant may accept a special request, so you can try asking the staff before placing your order.