One of the considerations in sizing pipe is that you want to observe the 5 feet per second speed limit. If water is allowed to flow faster than that, you increase risks of water hammer and damage to the plumbing. This is because kinetic energy quadruples as speed doubles.
This 5fps speed limit is where you are getting the number 23GPM for a 1-1/4 Sch40 PVC mainline. If you attempt to force more than 23GPM through a 1-1/4 mainline, you will be violating the 5fps speed-limit.
But just because there is a speed-limit doesn't mean your system can move that fast. As a ridiculous example, if your mainline pipe was 500' long and you had a static pressure of 10psi, there is no way in %#!! you are going to get 23GPM moving through that pipe.
So like Wet_Boots says, you've got to design for what your plumbing can produce, not the maximum allowed by "law".