Depends on the Supercharger, Engine & Pulley Used.
With the SC14’s Stock Pulley You are looking to make around 3 – 4 PSI on a 2500cc – 3200cc BMW Motor, Smaller Pulleys are available to take you to upto ~12PSI. For a Street Car with Stock Engine, Its Advised to have a Intercooled 8PSI Setup for Reliability,
NOTE: Boost itself varies and is much dependent on your motor, and any modifications done to it, as cams, intake, exhaust; then a good quality blow off valve setup right, use gfb/turbosmart/tial and well lube them, you dont want a stuck open valve bypassing all your boost and a good boost gauge itself, and finally its dependent on the supercharger drive pulley size used as well as the right belt. Another example the Stock 113mm Pulley produces around 12PSI on our tests in a M40 motor, while the same blower will produce average around 3-4 PSI on a M52B28. As Motor size goes up, boost goes down.
Easiest pulley is the 93mm one from the Toyota Corolla 2002 SRC Aircon, Some people even go as far as using custom made pulleys.