Is it good to place different advertiseon our web site or blog?

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HI friends I have seen some web sites where they place so many advertise from adsense, bidvertiser,adbrite,kontera and so many so I did not find that site god as there is less content and more content but they make good revenus so I did ot understand that instead of showing diefferent advertise they ca concentrate on 1 advertise sites so that they caneasily reached at minimum pay out.