Cubic inches, rpm's and gear ratio in the differential. Those will be the things to determine the size and quantity of exhaust pipes.
One 2 1/2" pipe equals about the same area as two 1 7/8" pipes, which should be enough to support evacuating the cylinders of a typical mildly-built 350-ish street motor turning street revs with a street gear.

In the past several decades, pipe size has seemed to go the way of camshaft timing, bigger, bigger, bigger.

In my opinion, a good set of 1 5/8" headers with equal-length primaries, 2 1/2" collector, H pipe immediately after the collectors and a set of 2" to 2 1/4" pipes to the rear of the vehicle with mufflers of your choice somewhere in the mix would be about as good as it would get for a 350-ish street motor rev-limited to ~5500. All this crap about 3" pipes, to me, is just that.....crap, unless you're evacuating a 500 inch motor at 7500 rpm's.
.