It is very much down to personal choice really, if you were to design the lighting correctly you would first have to establish the lux, lumen levels you require, and then view the manufacturers lighting specifications to establish how many are required. Dialux, or another lighting program can help with this. I guess you would not want to bother will all that, just get something that is nice and pleasing?
If you look at the packaging on lighting products you will see a cone shaped diagram, this will show you the overall throw of the lamps, this can be used to design your lighting, first find the height of the room, then when you look at the cone graph just decide how much of an overlap you need.