There are no hard and fast rules on this. Your question reminds me of a famous Yogi Berra-ism, uttered by Yogi when a waitress at an Italian restaurant asked Yogi how many slices she should cut his pizza. He replied "You better make it four, I don't think I could eat eight."
To be serious, 8 slices seems to be fairly standard for a 14-inch and 16-inch pizza. However, some pizza operators who sells pizza by the slice often cut their pizzas into fewer slices. I have read of 16-inch pizzas being cut into 4, 6, 8 and 12 slices. I have even read of 18-inch, 20-inch and 22-inch pizzas being cut into 6 slices. Some places that sell only by the whole pie will use 12 slices for a 16-inch pie.