Depends on the Supercharger, the Engine & Pulley used. It is best to look under the Our Products page or the Shop listings to get more information on each setup. Boost itself varies and is much dependent on your motor, and any modifications done to it, as cams, intake, exhaust; always remember to use a good quality by pass valve setup (not to be confused with a blow off valve).
For example with the V2 Kit (SC14 SC) with its stock pulley You are looking to make around 3 – 4 PSI on a 2500cc – 3200cc BMW motor, Using the HMW 80mm smaller pulley, you can see upwards of 6-10PSI. All while, with the M1 Kit (M122H SC), you would see 13-16PSI of boost on the same engine with its stock pulley.