400mm sounds good! Did you go for a smaller pendant?

I think it depends on the style and atmosphere you want to create in the room. If the room is a formal, traditional one like your picture, then one large showy pendant would look great. However if it is a modern or contemporary room then two ( or more) smaller pendants would be a nice feature. It would also pay to think about how much light you are expecting from the fitting, especially since it is a cathedral ceiling. Do you need the ceiling lights for good general lighting or just for atmosphere. Do you have any photos of your room so we can see a bit more?

It comes in three sizes, as well as only coming in the white. It'll be elegant over your dining table.

The number you need for adequate lighting is dependant on the light fitting, its brightness and the angle the light will cover. The physical size of the fittings will also affect the number that looks best. Two very small fittings may give adequate light but look lost, while three larger ones could seem crowded. You may even find some very small fittings that work well as a group of six. Take into account the width of the surface as well as the length so you can ensure it is all well lit. You may be able to change the brightness of the lamp in the fittings to give a good work light in the number of fittings that look best. Avoid creating strong bright/dark contrast areas on a work surface. More overlapping less-bright light is better than few brighter ones that leave dark patches between them. Also consider whether there are other elements in the space that the group of light fittings could relate to, whether window mullions, cupboard doors, seating etc. If something else is creating a strong rhythm, decide whether you want to echo, or offset that or have the lighting as a background element that doesn't have a strong relationship to it.