Generally the main factor in positioning sensors is the actual beam width of the sensors themselves. It's usually the case that the longer range the sensor is, the narrower the beam width.
So firstly you need to work out the range you are aiming to sense at and pick some sensors that work at that range.
Then you've got to decide on how wide an angle span you want to sense distance at. For instance, you may decide to cover everything in-front and to the sides of a robot (a 180 degree range, 90 degrees to either side of straight-ahead).
You then divide the angle span by the beam width of your chosen sensor to get the number of sensors you need.
Using the above example, if you have sensors with a 90 degree beam width, you'd only need two of them, each at 45 degrees either side of straight-ahead. If you had longer-range sensors with, say, a 20 degree beam width, you'd need 9 of them to cover the same area.
Obviously the beam width is not an exact measurement so its probably safer to have some overlap of the beams. Finally, the sensors should be installed symmetrically between the left and right sides of the robot.