In a complete circular pizza, we know that the central angles of all the slices will add up to 2π radians = 360°. Since each slice has a central angle of 1 radian, we will need 2π/1=2π slices or 6.28 slices to fill up a complete circle.
We arrive at the same answer if we think of this problem in terms of the pizza crust: we calculate the circumference of a circle is 2πr. Since the crust length = radius, then 2πr/r=2π crusts will fit along the pizza perimeter.
Now, if you are still hungry, take a look at the sector area calculator to calculate the area of each pizza slice!