Turbo and supercharger output, boost, is measured in either atmospheric pressures, (milli-)bars or pounds of boost. Most turbochargers stop producing boost after 20 to 25 pounds have been reached, although they are capable of much more. Superchargers, also capable of greater boost numbers, typically build up to a maximum of thirty pounds of boost. How much boost is the maximum safe boost pressure depends on how well your engine is built. If you have a stock engine, with a compressor that is factory set to produce no more than twenty pounds of boost, you shouldn’t modify the system to make it produce much more than 25 pounds of boost. And increase of 5 pounds over stock is probably the limit you should go by.
For more info, see Pros and Cons of Installing Twin Turbo Kits