We have 8 planets in this star system. All of them are spaced fairly far apart. Over the past few years we have detected a variety of extrasolar planets, only a few have been multiplanetary systems. This is more of a hit and miss detection method rather than one or 2 planets systems being common.
Two of the most recent discoveries are the systems HD 10180 and Kepler 11. Both systems have more than 5 planets. Kepler 11 is the system detailed in the photo at the star of the post. Note all planets are within the orbit of mercury with the exception of the last. This system is remarkable due to the fact that all 6 planets were discovered by use of the transit method. All of them passed in front of the star (from our point of view). Every planet has an eccentricy of less than 1 degree. Any other planet in system that was beyond the plane of 1 degree would not be detected. With all six planets within the comparable orbit of Venus, that leaves alot of room for other planets. I could images two dozen planets(24) or more if this magnitude of planetary packing was applied to others in system. Don't forget the 6 planets detected so far are at 1.5 + times the mass of the earth! This makes me wonder why our system is so sparse, especially in the outer solar system.
Several evolutionary causes of solar system density may be the amount of mass, how stable it became early in its history, or the environment around the star when it was being born. The amount of mass, more mass in the protoplanetary disk, more planets. If the system were to stabilize rapidly, ie no rouge planets migrating around the system, then then more protoplanets would survive to become planets. The environment around the star would determine how much accreation material would be blown away. These probably come into play interchangeably.
Kepler 11 is a wonderful system to think about, imgine if our system was as dense as it! It would certainly be easier to get two habitable plents in one system with a dense system like that. I sure would like a dozen more plants to explore!
HD 10180 is a favorite of mine. Partly because of its properties and mostly cause its an HD i can remember off the top of my head! It had 7 planets with only 5 confirmed. This one looks more like our solar system in orbit parameters, but look at the sizes!
Diagram Images from http://exoplanet.hanno-rein.de
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