"Some" guns do not require a break-in, others do. The ones I have seen with an actual number posted in the instruction manual is usually 200. Some guns have tighter tolerances and stiffer springs, others are loose as a goose.
Kahrs, for instance, are tight as Dick's hat band, and therefore require a break-in, and they recommend "high velocity" ammo to overcome the stiff springs and cycle the guns for the break-in. In 9mm, ammo like S&B, Fiocchi (~1280fps) would be high velocity. Ammo like Remington and Magtech (although good) would be low velocity (~1120fps). The low-powered ammo just can not run the slide in the tighter guns when they are new.