thehihopsoldier pretty much got it right,but have to agree with adam too.
more slave pistons mean more power if the master piston remains the same.
just force>>surface>>>pressure>>>surface>>>force if you look at the hydraulic system.but levers are a factor too.
but most brakes change master piston diameter according to how much slaves there are,so the power isnt thaaaat much more (sure is in most cases but not by times)but the modulation is increasing,because you have "steps" in the power diagram.the pistons move out depending on how you pull the lever and how far and so on.
hydraulic brakes arent really a simple press,with multipiston systems and features like servo wave a lot of factors come into play