How many Adsense accounts is one allowed to have?

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I currently have Google and will try to join Webanswers as well. I also want to know if I can add InfoBarrel as I just wrote 30 articles for them. Has anyone worked with InfoBarrel before? I have not yet received any money. What kind of earnings can one expect to make? This is a fast-changing environment and one has to keep up to date. Does anyone have something new to say about increasing earnings? Are there any new sites making a debut?