Thats because it depends on what you are trying to do with them, how high they will be mounted, what kind of lamp do you want in them.
When you say ambient light, if you mean general lighting, then you will need to know the rooms width, height and length. From that, the zonal cavity can be calculated and the CU applied and the amount of foot candles you want/need and the number of fixtures can be determined, and then the spacing. (for example, my living room I did 2 feet out from the wall, and 4 feet apart, at 7.5 feet, with a 50 watt halogen spot, and a dimmer gave me ample lighting).
You can also consider specialized placement. Like 1 over each chair where you will probably read, 1 at the door, 2 fixtures over the coffee table - 2 feet each side of the center, 2 by the stereo, etc. This you will need to look at each location individually. Consider a mulitude of patterns. row of 3, row of 2 ect, or a hexagon shape.