If there is less than 2 fingers, the saddle tree is too wide and may damage your horse's wither. You can usually purchase one in a good office supply store. Stand your horse square, on level ground. Carefully lift the curve off the horse's back, place it on a piece of paper or cardboard and trace the inside edge with a pen. If you take this to a tack store, try fitting the cardboard cutout under the pommel of the saddle.
Less clearance means the tree is too wide and more clearance means the tree is too narrow.
The conformation of the horse is not truly suited to carry a rider. Therefore it is important to have the weight of the rider distributed evenly over the horse's back. When a saddle sits in proper balance, the deepest point of the seat should be in the middle of the saddle. Check this by placing a pen in the saddle, it should roll to the deepest point of the seat. If the pen settles behind the midpoint of the saddle, the balance is cantle low.