1 die gives you a flat-line graph (on average). 2 dice give you a tent-shaped graph (points along two straight, diagonal lines that meet at a peak in the middle). More dice give you a bell-shaped graph and the more dice you add, the closer it comes to a true "bell curve".

A bell curve is defined by the shape, not how high or wide it is. Adding more (regular) dice makes the resulting values larger. Instead you can use balanced dice that give values from -2, -1, 0, 1, and 2, for example, or -2.5, -1.5, -0.5, 0.5, 1.5, and 2.5.