How do g-suits help in minimizing the effect of high g's on the pilots?
G-suit create extra external pressure on the limbs (legs in particular) to prevent the blood to stagnate far from the heart. In normal conditions specific biological systems automatically return the blood from the extremities to the heart, but when subjected to high G loads these systems fail, as they are not designed for such conditions.
By creating pressure on the body the G-suit alleviates the G load perceived by the body.
What is the maximum g that a g-suit could handle properly?
The main limitation is not only the amount of Gs, but for how long the human body can sustain them (spoiler alert: not much)
As you can see the Gs usually experienced in a fighter aircraft (blue line, see diagram on top right) cannot be sustained much beyond 3 seconds if they are of even moderate intensity. The G-suits help the body resist slightly longer.