Maximum pitch angle would totally depend on the type of helicopter you are flying. Each helicopter has its own set of limitations and pitch angles depending on the blade, rpm, etc. The controls include a mechanical stop, and you would hit that limit sooner or later if you put in an overly aggressive control input on the cyclic, or full collective up or down. Think of it as the same way you would hit the outside edge of the control on a video game joystick.
In "real life", the pitch angle numbers are irrelevant to a helicopter pilot. During
autorotation, the RPM of the spinning blades is maintained by reducing the pitch angle allowing an upflow of air from the descent to maintain the rotors spinning - similar to blowing air on a fan in order to keep the fan blades moving. The rotor RPM can be adjusted through up and down collective movements, which will in turn increase or reduce the RPM required.